Specifically the kick check has to block your opponent's snapkick using your leg.
By contracting your knee and using your bent leg to shield the strike you can deflect the kicking force and unbalance your opponent.
This came from muay thai and is super effective in blocking and can even injure the kicker.
(Look at anderson silva's break against weidmen's check.)

This is from my replay thead and should help you study the kick check better:
http://forum.toribash.com/showpost.p...2&postcount=17