The only sheet that is there is that he is trying to bullsheet people into believing him
An older video from his Instagram:
Basically, he is using his diaphram and abdominal muscles to extend his belly outwards. Opposite of the yoga abdominal vaccuum where one sucks the belly in.
I learned to do both innately as a young child, then learned that it's call 丹田 breathing when I picked up martial arts in my teens.
Thanks to that, even though my 10-pack is now hidden behind a layer of blubber from the good life, my core is still hard as rock.