What's behind the "hairdryer" fix for broken iPhone 4s WiFI?
I have tried this fix and it does work although it wasnt permanent.
The only thing I can think it is doing is re setting the base temps of the motherboard and chip, much like a PC. In other words the software might be looking for the difference between the motherboard and the chip temp. 
If they have set this too low for the 4S then obviously it would show it is overheating even if it isnt.
By heating up the motherboard and the already hot chip (which wont heat up further as much) you are re setting the parameters and so it works for a while or permanently. I'll try further.