It's about girl being fine with ATM throughout the scene, but at some point starting to feel uncomfortable about it. I saw it few times in LP movies. It happens for newcomers, that forget to spit after ATM. Given enough dose at short time and deep ass pounding (never seen it from tipfucking), girl melts down a bit. Not enough to pause the scene, but enough for you to notice.
A great example you can find here: Black Monster Cock Junkie Samantha B Video SZ127, starting around 36:30 mark.
BTW, what is unfortunate about this scene, is that brunette girl, hero of the scene, is unnamed here. Appreciated if anyone recognize her. Asking people on internet about girl name, feels 2010ish

I find this kink very hot, as it shows newbies struggle at their first movies. Do you know others LP scenes with this kink?