To slap your partners forhead with your cock while cumming during oral sex.
To perform an MC Hammer you MUST be standing up- UNDER THE CONDITION that your partner does NOT want you to cum in her mouth.
Right before orgasm you shake your legs and say "uhoh! uhoh, uhoh-uhoh! STOP!" - Pull out - Begin to bang your dick on her forhead while cumming and say "HAMMER TIME!"