Etymology of "vinegar stroke" (TMI, likely)

I’ve heard the term “vinegar stroke” in reference to the last stroke before ejaculation. Why “vinegar”?

I believe it refers to the strained facial expression - as if one were tasting vinegar.

Ah! That makes some sense, thanks.

Ah: A Viz-ism. I’ve always taken it to refer to ejaculation: a visual allusion to shaking vinegar from a bottle (the narrow-spouted kind used in the UK to sprinkle little squirts of vinegar on chips).

More like G-ism.

NOT a good visual being imaged here…