When you've typed your password on your keyboard countless times so that your not paying attention and it's in your muscle memory, that's phantom fingers.
When you've done your locker combination so many times that youre not entirely sure what the combination is but you know how your hand moves that's phantom fingers.
Kid A: "So how did your date go with Lisa?"
Kid B: "It was fine. I fingered her, but i've done it so many times I've just got phantom fingers for her man."
Kid A: "...I could do it for you..."