When you take a towel and hold on to each end. Putting the towel in between your legs and moving it to the front and back (like you are flossing your teeth, but instead its your balls/crotch)
My balls were dripping wet after a shower so I towel flossed them.
It's when you take a massive dump over your towel. You wait for it to become dry and crusty so you can wipe it off the towel so you can sculpt it into a anime body pillow.
Mom: Niggato what are you doing in there?
Niggato: I'm masterbating to my towel feces! Don't look!