Some of the ladies who do/allow this will initiate it while they maintain control by being on top (cow girl). This will allow them to not only control the force/pressure and back and forth distance, but also gives them an escape route should you attempt to sneak it in!
After you build a bit of familiarity and trust with the lady, she might allow more pussy sliding (AKA P2P) while in other positions, such as missionary. It can be a quite erotic activity, building the anticipation for the main deed!
NMBob