This situation happens often. A lot of people email me about it, and so i am posting a technique article. a good stack against a leg lock can help escape it, but a mediocre stack against a leg lock will only make it worse against an experienced grappler. if you want to avoid him standing over you in the first place then simply control his other leg (see my article on leg control.)
your opponent stands over you (because you entered into leg control standing or because you did not control his other foot!!! )
the knee down stack
your opponent places his knee on your chest in hopes of discouraging your leg locks. he can grab behind your head and squeeze all he wants. as long as his knee is on your chest you can still submit him.
grab foot and turn
grab his foot and push it away. at the same time turn your hips toward his other leg.
lay on your side
if you squeeze your knees tightly and you lay on your side then he will have to lay on his. now yo ucan do the toe hold...
and the heel hook.