Portuguese Nutcracker
When having sex with a girl doggystyle, before you ejaculate you remove your penis and stick it in her anus, while at the same time try to stick both of your nuts in her vagina. This is physically impossible due to the tug to cum ratio of the nuts.
I was having sex with my girlfriend last night and I tried to give her a Portuguese Nutcracker... nearly thought my balls were going to fall off.