When you are doing missionary, you insert your finger into her bum and can feel both your penis and the finger against each other to find the sweet-spot, like a lockpick.
I was doing missionary with Alissa the other night and did the lockpick. Gotta say, shit was tight.
The act of having sex with a chick and either her or you sticking 2 fingers inside her vagina at the same time as his dick; hence, it is like picking a lock.
Dude!! This chick I was fucking last night was totally 'lockpicking' herself but after a few minutes, I took over.