Why Belly Heat is Necessary

[tHeGiNo]

I thought this would be an exceptionally interesting thread, as I have often heard that snakes necessitate heat from their belly (ie. heat pads). So basically, here is what I am questioning: to 'bask' a snake retreats to a rock, tree stump, tree branch etc. under the sun. The sun was hitting this spot directly and is therefore heated. However, once the snake is on this particular area, is the heat not absorbed? It would make sense to me that once they are on the particular basking spot, the sun is no longer hitting the item in question, and is now hitting the snakes back.

My understanding, actually my reasoning, behind this subject is that if they require belly heat, they move to a different basking spot once the heat on the surface they are basking upon is depleted. Is this the way things work?

I am seeking some clearance on this, so that I may have a better understanding. Thank you in advanced for all your replies, and lets keep this informative and lacking any name calling.