Having studied human sexuality for years, it's not often these days that I am surprised and delighted by something new. I don't know how I didn't come across docking until now, but I'm so glad I did because it is truly enlightening.
Docking is the act of slipping one foreskin over the glans penis (dickhead) of another. I could try to explain further, but there's really no better way to understand docking, than searching it on a porn site. It's a real aha moment.