The man needs to have a shit that is about the same size as his erect penis. Remove the said shit from the toilet and put it in the freezer for later.
Once your woman is home remove said shit from freezer and place under pilow (or easy access to wherever the act is going to take place).
Once you and your partner are engaged in sexual contact you begin to have anal sex. The man teases the woman by coming all the way out and back in again. After about the 4th time, stealthily grab the shit and shove it in her anus.
Whilst she still thinks your inside her stealthily run up and turn on the light. Run toward her shout "Ninja" and punch her in the face.