I while ago in Denmark we were training a knee-tap to direction change (to double leg or something else) stakedown series which has been used by GSP multiple times fighting in the UFC. The general idea of these direction change combos is that your opponent will have a much harder time keeping his hips heavy when the angle of the takedown pressure is changing constantly.
I was also told the knee tap is much harder to do when your already clinching with your opponent. Instead in mma it should be the entry takedown when your opponent goes for a jab or a cross.
Here is a quick video on MMA trainer Carsten revising the combos to me through a foggy lens
Here is a compilation video of GSP doing it in his fights.