Many of the rogue's powers function only when the rogue has momentum.
You gain momentum by hitting an enemy with an attack.
You lose momentum when you are hit by an attack.
The default is that you can use momentum powers without losing momentum, but a few powers specify that you must spend your momentum to use them. You don't have to use attacks that require momentum against the foe you hit to gain that momentum.
Momentum powers that do not require you to spend your momentum are generally classified as interrupt actions. You can only use one interrupt action a round, which keeps your momentum powers from dominating the battle.