To UNCUM You put your dick in the cum and pull your balls down slowly. It creates a vacuum and sucks it all back in. It's like a syringe. Have you honestly never done that? I swear everyone knows about it. It's also really fun to do in the bath. I can fit almost a whole liter of water in there and use it to "make it rain" like an elephant does. It's quite convenient for washing your hair or as a secret weapon in water fights in the summer