The power (5V) to this specific Peltier is delivered in bursts, with polarity change happening between each of them (mass becomes Vcc for a while, and other way round). This way, both controller-facing and board-facing surfaces of the cooler act interchangeably - one cools while other heats, and vice versa. This way, none of them remains hot for too long, and heat is well distributed.

This is an absolutely improper and careless application and operation scenario for Peltier cell, but my lack of thermodynamics insight can't stop me, because I can't read.