Arbiter

You're always the first to start the battle and always the first to finish the war. You've learned to survive, aligning the stars to your desire is the best way to survival.
Though Arbiter is one of the most mobile perks, it's also very good at Crowd Control. The projectile boost can be used to clear out the stronger, while the left over knuckleboom can be used to clear out the weakest. The knuckleboom may seem weak, but it can be extremely helpful when you need to prevent an unparriable attack from landing, as pressing F is a simple toggle, and can be triggered in an instant.

[Fanmade]
The Augmentation 'Knuckleboom' and 'Knucklefist' is, hence it's classification, an Augmentation. Usually, the Knuckleboom is only used, because the Knucklefist is heavy, which can heavily slow down an Arbiter. However, a certain Arbiter has chosen and learned how to lift the weight of the Knucklefist, utilizing to great use.
While the Knucklefist can create explosives out of the pressure and momentum put into it, the Knuckleboom doesn't have Melee Capabilities. The Knuckleboom (which we will call the Shortshot, the technique coined by Agents) can only create the blast out of simple pressure, dealing much less damage than the Knucklefist. However, it's enough to assist the User in good crowd control.
The next technique however, it's not too complicated but also requires timing. Coined as the Longshot or the Projectile Boost, Arbiter Agents have learned that flicking a Shotgun Shell and timing their Shortshot will direct the Shell to wherever they're aiming and create a powerful explosion. This is one of the many reasons why most Arbiters only choose to use the Knuckleboom.

Note from an Agent - The Knuckleboom can use the momentum if the Knucklefist isn't installed, and they both share the same system, but the momentum can only be focused onto one thing, and isn't able to support both capabilities. That's a pretty big reason why most of us don't use the Knucklefist at all.