Melting faces describes the action of a boot camp instructor or personal trainer whose ability to influence the body and mind is so powerfully hypnotic that you could literally keep moving until your face melted right off your skull.
client1: how many more reps do we have to-
Instructor: Less talk!!! More push!!! Let's see some melting faces!!!
client1: Aaaaaghhh, I'm meltiiiiiing!!!
client2: Ahahaha, she's mel- oh, WTF, I'm melting toooooo!!!