The facial expression during sex that coincides with the last thrust before coming.
Imagine the contorted 'fuck face' being pulled by the male as hes about to come, and compare to the wincing one would experience when sucking a lemon, or tasting vinegar. Hence, the vinegar stroke.
Ah man, I was banging this chick, and I was up to the vinegar stroke when my phone rang. Ruined the moment.