This can be assisted by leapfrog but can also be achieved single-handed. The push through is a strong border push up the centre by a pocket player with the specific intent of placing a castle, or other fortification, in between two of the enemy territories. Cities are built in a straight line with strong borders, looking for a weak spot between enemy nations where a castle can be built to push apart further. If the push is far enough (and it can even completely divide enemy territory, then you have an excellent position to attack from. When enemy land is divided, trade caravans will be shot up by castle and tower fire and are subject to attrition as they cross the land.
This strategy is good for supporting an attack down the flank as it has the ability to isolate a flank opponent. Any supporting units coming from the enemies pocket player will first have to navigate through your territory and past your castles and towers. There are good ambush possibilities as well as using your army to help the attack down that flank by conducting attacks into the enemy economy, either with significant raiding or more significant capital siege.
iving a wedge between the enemy like this is likely to attract a double attack from your opponents. It is therefore important to ensure that your adjacent flank player is militarily strong enough to either take this double pressure off or take full advantage of an absent army on the flank.
A pocket player aggressively pushes his expansion through the enemy position while the nearest flank goes strong military to take advantage of this isolating disruption.
Used with three or for player teams on a land map, preferably one where the map size is larger than usual for the number of players.
This strategy takes much of its strength from forcing the fight in the enemy's back yard and by giving another team mate a significant advantage.
Risk: Medium
Effectiveness: Medium