Pretty much every time I've noticed this, it's when the guy is standing with his penis near (or against) the girl's face after the cumshot. A few examples:
Pretty much any gloryhole secrets video usually has it. The girl lets him go too soon, and usually a guy is left there still shooting or dripping. Can't name any specific girls, but i'd recommend renting any.
1st cumshot at 21:48 and he steps away while the other dick moves in to jack over her tits, at 22:03 the first dick moves back in and lets a few more drops of cum hit her chest. 2nd nut all over her tits at 22:19
That was hot. I like that he was able to push out those last few drops by stroking his dick—he didn't have to shake them out, or pinch the head.
This video is almost the reverse of that. At 1:58, he moves toward Echo Valley's tits, his cock drips before he gets there, and then he shoots a little load of precum on them. He strokes it for a while longer, and some more little drops of precum fly out. Finally, at 2:35, he jizzes: