Which animation cancel? There's tons.
Ability animation cancel: You chain a bunch of abilities with animations together, followed by a single Q. For example, R (first cast) + Hydra + W + E right at the same time. All the animations get cancelled except the last one in the chain.
Q animation cancel: This is the tricky one. You Q, and while mid-Q, you click somewhere away from yourself. Then as soon as you land, you click a target to autoattack, essentially cancelling the autoattack animation.
Video reference:
https://www.youtube.com/watch?v=0qO715z_OKk
Autoattack animation cancel: essentially Q animation cancel in reverse. You autoattack, and immediately after the damage has been dealt, you press Q, so the wind-down of the autoattack animation is canceled and instead replaced by the Q animation cancel.
Riven fast combo:
This is a combination of all the animation cancels. R (first cast) + Hydra + W + E + autoattack (autoattack animation cancel) + Q (Q animation cancel) + autoattack (autoattack animation cancel) + Q (Q animation cancel) + autoattack (autoattack animation cancel) + Q (Q animation cancel) + autoattack + R(second cast)