I love watching that too. But only when the lover ejaculates with his cock thrust balls-deep into the vagina and holds it there until the muscular throbbings in his pump-bulge and root completely cease, as my young black buddy invariably does when he finishes in my ex-wife.
The "held-backness" (most likely for the video-cameraman) of the only half-buried ejaculating cock captured by your GIFF must have been incredibly frustrating for the woman. And her lover too.
The extremely arousing spectacle of the back-spilling of semen is never forgone by me, because my buddy finishing balls-deep results in his ejaculations being very forcible and all the more copious and---from my observing them close-up---satisfying for him and his wife (my ex).
The visually beautiful bonus is that my buddy's semen, being pumped back along his orgasmically jerking inthrust shaft, erupts as a thick white collar appearing between his buried root and the maximally stretched rim of his wife's orgasmically contracting vagina.
Even more thrilling for me, when my buddy eventually withdraws his still almost fully-erect cock, the long-persisting orgasmic contractions of his wife's vagina cause much of his til-then un-erupted semen to well back and overflow from her gaping opening.
Of course all of the above adds mightily to the incredibly arousing spectacle for me as the intensely voyeuristic ex-husband, and my wife and my buddy are of course fully satisfied too.
All because of my buddy ejaculating not half-cock, but uninhibitedly and very powerfully balls-deep.
Everybody wins.