How to animation cancel? I have never heard about it.
Attack animation like crossbow has the following
Wind up time (Get into shooting position)
Shooting time (Bolt flies out)
Recovery time (Putting down the crossbow, can't do anything)
The recovery time is insanely long so you can only get 1 shot out most of the time before the combat begins. However, if you can remove that, you will be able to wind up and a second shot and send it flying as the fight begins, giving you another hit on the enemies as the bolt is already in motion, or even 3 if they take ages to find you.
Simply attack an enemy and the moment you hear and see the bolt fly out, going into sneak mode. The sneak mode can be used to delete the recovery animation and allow you to do issue another attack command while the first bolt is in motion.
It's extremely strong, even stronger than using Snipe with sneak or barrage. Only really needed if you want fights to be really easy or having issues keeping up with gears and need a bit of extra damage.
