Usually I just stick to a quick (and I mean fucking quick. Think almost vibration speed. It takes practice. If you are a drummer, God is begging you to try this) penetrative, in and out motion, palm down, with my middle finger (add the ring finger too if you think it will fit well enough to not take away form the speed) if I am transitioning to this during oral. If I pull out and do this, I usually like to lick from her nipple to her neck and lightly bite her there right before she comes as I use the same real quick, in and out, penetrative motion this time with my palm up using more forearm muscle to create the vibration effect than wrist. You can also try experimenting with different amounts of pressure on the clit under there while doing this; I have noticed that just barely brushing it works best
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So in review haha, implement this right as she is about to come (which takes some guesswork and experience to pinpoint with new girls, but it isn't particularly hard to hit the correct window). Shift from quick to slow, deep strokes, then pull out and put in the finger/fingers and finger as fast as you fucking can. My theory is the sudden shift from deep ass strokes to quick, shallow vibration type strokes is what sends the girl over the edge, but truthfully I have no idea, I just up and tried it one day, and it worked.
Note: Licking your finger/fingers also helps with the speed.