Luiz Panza is one of the best foot lockers in IBJJF Competitions. Attacking the feet in BJJ competitions is typically not seen as often in limited rules no gi grappling.
There are 2 reasons for this. One, there are limited rules for foot locks in IBJJF tournaments, yet the straight ankle lock you can do at blue belt in above. The second reason is that it is actually much harder to attack the foot with the gi, due to the gi itself. It gets in the way, your opponent can grab more things, and there is more friction.
Luiz uses the gi to actually finish the foot lock to create the pressure a lock against his bone right above the heel.
The foot position of Luiz is also very important. He keeps his feet right on his hips and squeeze / control with his knees.
The belly down foot lock is so tight and powerful, which is evident by how many straight ankle lock submissions Luiz Panza has hit in IBJJF competitions.