While maintaining Uddiyana and pressing his hands into his thighs, Yuha gives a downward and forward stroke to the rectus abdominis, just above the pubic bone. Contracting the recti and pushing them forward, he isolates the recti from the other muscles. This is Nauli-Madhyama or Middle Nauli.
If we could slow down this video, you’d be able to see that Yuha holds the middle position for a second, then he applies more pressure on the right thigh with the right hand, giving a greater bend to the right side of his body. At the same time, he relaxes the left side. This keeps the right rectus contracted, rolling it further to the right while left remains inactive.
He goes back to the middle position before pressing on the left thigh with the left hand while relaxing the right side of his body. The rectus rolls to the left. Repeating these actions quickly, right-middle-left, right-middle-left, over and over, gives the appearance of churning the abdominals.
After churning five times one way, he churns the other way, thus performing Nauli kriya.
Slowing down to a more controlled “churn” produces different effects. For me a nice slow nauli heightens the sensations in my abdominal viscera, activating my enteric brain and sharpening my intuitive skills. Try to observe your abdomen, and note how you feel before and after the practice.