There are two things that a ball python uses as feeding signals, scent and heat signature. All living warm blooded animals have a heat signature. When you drop a dead rodent it will have a scent which it recognizes and then it will go for the heat signature. This is how folks get bit sometimes when feeding because your hand has a heat signature and if a snake smells rodent it could mistake your hand as lunch. That's why he is striking at the heat.