Vinegar stroke

[Mark Zuckerberg]

The point during [sexual intercourse] where a man is irreversibly about to [blow] his [load]. The very worst time to have to cease [coitus] in an emergency.

[PussyCat]

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.