The transformation from a guys casual facial expression to the one he puts on right before he goes for the pink cave.

A mixture of a fake smile and complete fear.

Fake smile because he promised he loved giving oral
complete fear for what he might find and/or smell

Between bros and dudes it is often refered to as PLF

It’s an acronym standing for Can’t Remember A Fucking Thing. It’s usually used to describe embarrassment at being unable to remember someone (CRAFT moment); going onstage as the lead in the latest theatrical performance and your mind goes completely blank (CRAFT situation); the next morning after a night when you got pissed as a fart and woke up in bed cuddling a traffic cone and a string of fairy lights wound round your prick.