I must have missed something. Reveal or not, I want to know why he not screaming in pain while his fingernails melt off? It is still fire. Obviously I have never tried this.

not sure how it relates to exposure?

the heat (particularly at the base of a low intensity flame such as in the video), that takes a few seconds to conduct heat into whatever is next to it (his hand). By keeping the flame ball in movement, the heat is not being conducted into his hand. If he kept the ball still long enough he will get burned. He is definitely feeling some pain in some parts of the video (particularly when he closes his hand) and when he does he starts moving it again.

Similar principle to firewalkers.