How do you check a low kick in UFC 6?

Both triggers — L2+R2 or LT+RT — as the kick lands. That is the low block.

Why do I still eat the kick with the triggers down?

You were early (feint) or late (the kick already landed). Drill the start of the animation in Practice Mode.

No. They will feint the kick and throw the head. Check the real ones.

A check is a low block that meets the kick with your shin. Early and they feint you. Late and you take the kick whole.

How to check a low kick in UFC 6: both triggers on the impact, and what to throw after a clean check.