Pow! Hand Extension is not as ubiquitous as Rocket's Admin., but it has earned its place in many of the top decks in the format. They either play the maximum 4 copies of Pow due to its sheer strength and how often they want to draw it, or 1 copy for its utility with Pidgeot to search it out when needed. You either play Pow yourself, or build your deck to resist its effects.
Pow is several things: it's a comeback card, and it's a forced switch-in. You can click those links to read about strategies regarding those properties of Pow. Its other effect is moving an Energy card attached to the Defending Pokémon to one of your opponent's Benched Pokémon. Under the right circumstances, this can be as good as removing that Energy:
You can move Double Rainbow Energy or Scramble Energy onto a Basic or ex, causing it to be discarded. (Magic players will have to pay attention to this one: you can declare an illegal target for those Energies, and Pow will not be countered; the Energy will just fall off!)
You can move the Energy to a Pokémon with non-threatening attacks, like a Jirachi left over from setup.
You can move the Energy to a Pokémon that already has enough Energy to attack and doesn't need any more.
You can move Double Rainbow Energy to a Pokémon that already has another Double Rainbow Energy attached, weakening its damage output.
You can move Multi Energy to a Pokémon that has a Special Energy attached, causing it to provide only Colorless. Getting two Multi Energy on one Pokémon is the dream, as they will turn each other off.
No matter where the Energy ends up, Pow's Energy moving mode reduces the amount of Energy on the Defending Pokémon. Sometimes you don't have to hunt around the opponent's Bench for Energy-less Pokémon to trap with Pow's forced switch mode; you can use Pow's other mode to move Energy off their Defending instead, and they'll end up with an Energy-less Pokémon Active all the same.
If you have two copies of Pow in hand, you can mix both modes of Pow to create devastating plays:
You can use the first Pow to move Energy to a weak Benched Pokémon, then use the second Pow to switch it in and KO it. Weak evolving Basics that your opponent has already committed Energy to in preparation of future attacks make juicy targets.
You can use the first Pow to switch in a Benched Pokémon with low Energy, then use the second Pow to move Energy off it. This is useful if your opponent's Active has enough Energy on it that two Pows, both moving Energy off, still leaves it able to attack — if one of their Benched Pokémon is more vulnerable, go after that instead!
Minun's Sniff Out or Rocket's Wobbuffet's Dark Aid allows you to reuse Pow. This can create a deadly lock. First, you use Pow to force a Pokémon that doesn't have enough Energy to attack or retreat to the Active Spot, then return Pow to your hand with Sniff Out. Subsequently, if your opponent attaches any Energy to that Pokémon to try to pay for attack or Retreat Costs, you Pow the Energy away. As long as you're using Sniff Out every turn, you can match their 1 Energy attachment per turn with 1 Pow removing it.
Pidgey's Corner prevents retreating, so you can Pow in a Pokémon that doesn't have enough Energy to attack (even if it has 0 Retreat Cost, like Latios ex δ) and keep it trapped with Corner while you build up your board.
You can Pow Energy away from your opponent's Active Pokémon, then bring in a Pidgeot with 2 Energy attached and chip away at it with Clutch while it's unable to attack or retreat.
Note that your opponent can escape with a switch card, or by evolving their Pokémon to get rid of the "can't retreat" effect.
If you've been on the receiving end of a Pow deck (and it won't take long if you're new to the format), you might be wondering how to beat it. These strategies will prove useful:
Links: switch cards, Energy movement. The Mew ☆ package is also worth trying to find room for.
These cards counter one mode of Pow each.
Switch cards are mostly Trainers or Warp Energy, and can be added to any deck easily. Energy movement Powers are usually found on Stage 2s, so they must be heavily built around; it's more a case of you choosing a deck with Energy movement in it if you're not in the mood for losing to Pow, than trying to fit a Stage 2 line in your brew. Even Mewtwo δ has to be built around, as it can only move basic Energy.
How do you stop your opponent from forcing in Pokémon that can't attack or retreat with Pow? By not playing them in the first place. If a certain Pokémon usually forms a core part of your gameplan, but you don't need it to beat the Pow deck, then don't play it.
Example: you're playing Meganium, and you have an unevolved Bayleef with 0 Energy attached on the Bench, with Meganium ex and Mr. Briney's Compassion in hand. Should you evolve Bayleef into Meganium ex? Unless the situation is desperate (you can't win without Nurture and Heal, so you have to evolve and pray that you fade Pow), probably not, because they can Pow Meganium ex in, at which point you can't use Briney to get it out of the Active Spot because it's an ex.
Note that sometimes your opponent will have Unown E to make you put down Pow targets. Against that, try to discard those useless Basics to your Supporters, and play Holon's Pokémon as Energy whenever possible to get them out of your hand. Don't Holon Mentor for a Holon's Pokémon unless you're going to play it on the same turn, and don't return Holon's Voltorb/Magnemite to your hand when you play Holon's Castform.
Scoop Up effects, Swoop! Teleporter, and Giant Stump can be used to get rid of Pow targets clogging your Bench.
A Pow-resistant board.
A vulnerable board.
Attaching Energy when your opponent's Pow is live (or is going to be live) should be done carefully. If you commit too much Energy to one Pokémon, your opponent can Pow it in and KO it before it gets a chance to use all that Energy, or they can Pow in another Pokémon that you neglected to attach Energy to and make you waste resources getting it out.
Consider the two board states to the left. The one with two Benched Banette ex's each with 1 Energy can weather Pow better. If the opponent Pows an Energy away from the Active Banette, or Pows in a Benched Banette, you just need to attach an Energy to the Active Banette to continue attacking. Worst case, if you whiff the attachment, you discard an Energy to retreat. The other board is more vulnerable: the opponent can Pow in the Benched Banette with no Energy, forcing you to spend an Energy attachment retreating it. Worst case, you whiff the attachment, and can't attack for a turn.
Pow moves one Energy card, but if that Energy card provides 2 Energy, it becomes all the more devastating.
Whenever possible, try to attack with Pokémon that don't have double Rainbow Energy attached. If, say, all your Benched Pokémon are ready to attack or retreat, and the best your opponent can do is move 1 Energy away from your Active Pokémon with Pow, you can attach another Energy next turn and continue attacking without missing a beat. If your opponent can move 2 Energy away with 1 Pow, that'll be a bigger problem.
Suppose the worst has come to pass, and you're stuck with an Energy-less 2+ Retreat Cost Pokémon in your Active Spot. How should you proceed?
To answer that question, consider a scenario from a Bombtar/Scrambled Eggs deck's perspective. Their plan is to advance their board slowly with Jirachi or Exeggutor's Psychic Exchange, while they set up a big turn of Extra Energy Bomb to attach Energy to their attacker, attach Cessation Crystal to said attacker so you can't Quick Search for an answer, hit you with Rocket's Admin. to further limit your options, then go to town with their attacker.
When you get trapped, you should be thinking along the same lines. Figure out what you need to do, and try to do everything in one turn. The main thing to realize is that even though your Active Pokémon can't do anything, your opponent can't really make things worse for it with Pow. If your Active Pokémon has no Energy on it, Pow can't move Energy off it, and if your Benched Pokémon do have Energy on them, your opponent won't Pow them in and give you the first hit for free. So your plan might be to get Energy onto a Benched attacker, then use a switch card to get the trapped Active out, maybe also hit them with Admin and hope they whiff Pow on the redraw, then attack with the Pokémon you brought in. Of course, your opponent isn't going to make things easy for you, since they will probably be spreading damage to or Energy Removal 2-ing your Bench at this point, so you are on a clock to get everything together.
The idea of the "big turn" is an important one to keep in mind. There is less room for your opponent to keep you stuck if you can perform all the actions that you need in one turn, chief among them attacking. Remember, your current situation is that you cannot attack; if you perform a series of actions that doesn't end in you attacking, and your opponent subsequently traps something that can't attack again with Pow, you've effectively made zero progress, and also wasted a bunch of resources. Far more preferable would be to perform a series of actions that ends in a successful attack; even if your opponent does end up trapping something again, at least you've made some progress in damaging their board. If you play the switch card and bring in an attacker without enough Energy to attack yet, thinking you can just attach an Energy next turn, they can Pow Energy off it, or Pow your previous Pokémon in, and you'd be back where you started. If you attach Holon's Castform to your Benched Pokémon, but don't have the switch card to get it in, your opponent may have Energy Removal 2 to discard Castform before the attached Pokémon gets a single attack off. You don't want to do all this stuff that your opponent can easily undo, at least not without getting an attack in for your efforts.
Beating Pow is hard — it wouldn't command the reputation it has if it were easy — but if you keep a cool head and watch where you're playing your cards to the board (and also whether you should even be playing those cards to begin with), you can do it!