Seems some folks advocate for a quick heat, and im the only one saying do it slow. I guess I will explain why i say do it slow so i dont look stupid.
With a slow heat, its harder to overheat and burn the belly. Also, the heat soaks in deeper. A good deep heat treat is always ideal. Now of course you want the back to stay cool and thays what the damp rag is for. Of course that only helps so much, so you dont want to heat as long as you would without sinew. Just keep feeling the back and when it starts to get to the point of it being uncomfortable to hold your hand there for a solid min, you are at the threshold. When Im done putting heat to a belly, I take a wet not damp, rag and wipe the belly. With the belly facing up ( heat rises ) to cool it off as fast as possible. I feel doing less than this just doesnt make the effort worthwhile, the belly just doesnt get heat treated enough.