For example:
Flowing Gale Hook- you either play 2 attacks or you cant play any for free (correct?)
what about cards such as
War between Sisters: R commit: After your opponent plays an attack, both players discard 2 momentum and cannot add any cards to teh momentum during this attacks enhance step

If I can only discard 1 momentum do I discard none?

If I'm wrong with FGH can someone point me in the direction of some cards that do force you to do all or none?

says may, meaning you can fufill it by either playing 2 or none. you're not forced to play 2 so you dont do as much as you can

Ziepnir hit it. That's exactly why "do as much as you can" doesn't apply to FGH. It's the exception, not the rule.

what about the second question? for war between sisters, is it also two momentum discarded or none??

you discard 2

rC( ) said:

haha, I don't think that's what he meant. I believe, he means to ask if it is all-or-nothing.

And yes, the effect resolves as much as possible. If you have one momentum you still discard it.