Ever heard (or been part of) one of those endless old debates, like how best to survive a freefall in an elevator? Do you jump at the last moment or do you lie down or what? I for one, think that it won't matter because you'll be disoriented bouncing around the floor and ceiling as you are free-falling, so you wont be in any position to jump or lie down.
Another of those famous questions involves the best way to get hit by a speeding car? Lots of folks propose jumping at the last moment, so that you bounce over the top instead of getting hit in the legs and driven over. This one makes more sense to me, though I bet you wouldn't be in very good shape after bouncing over the top of the car either.
A similar phenomenon happens in ukemi - especially for judo, and especially for throws like osotogari and taniotoshi. It seems that the most common beginner falling problem with these throws is the desire to keep the feet on the ground instead of letting the throw clear your feet and turn you over for a nice landing.
In osotogari, for instance, Tori the Tank sails in and kicks the everlovin' shnot out of the back of your leg, and for an instant there, you're not sure if tori has enough to actually make you fall, so you strength-up and put some weight on that foot. Problem is, even if Tori the Tank does not have enough oomph to throw you cleanly, he often does have strength enough to buckle your leg and crush you into the ground. When this happens, he often gets his leg entangled with yours and falls on top of you in a heap of twisted, mangled legs. I can think of nothing that makes me cringe in judo more often.
Then there's taniotoshi. This thing is fairly gentle and soft when thrown by a proficient tori with a compliant uke, but as soon as uke sticks his foot and shifts weight onto it, there is this awful torque in the system that is a veritable machine for breaking knees!
The solution to both of these throws, and a lot of similar problems, is (listen up) uke, GET YOUR FEET OFF THE GROUND AND TAKE THE FALL!
The only way that you'll develop presence of mind enough to get your feet off the ground and take the fall when hit by surprise with one of these terrible leg-benders in randori or shiai, is to take a lot of falls like that during cooperative practice. When you are doing nagekomi practice (trading throws) and you know ahead of time that you are going to be uke, then you know that you'd better GET YOUR FEET OFF THE GROUND AND TAKE THE FALL!
It also helps a lot to add a couple of rounds of okuriashi falls - just like in nagenokata - to your warmup/ukemi time at the beginning of each class. Okuri is almost never a leg-bender (though it can twist ankles), but it can suffer from this same foot-stick, and this is a good chance to start learning to fix that foot-sticking resistance problem.