According to the website,
The Rape-aXe system consists of a latex sheath, which contains razor-sharp barbs. The device is worn in her vagina like a tampon. When the attacker attempts vaginal penetration the barbs attach themselves to the penis, causing great discomfort. The device must be surgically removed, which will result in the positive identification of the attacker and subsequent arrest.If this is failing to compute for you they have an explanatory flash video here.
Apparently they were originally called RapeX but discovered the name was in use in Europe already so changed to Rape-aXe. Personally I think it's a far more applicable name.
Now please excuse whilst I walk outside gingerly for a cigarette and allow my boggling mind to find itself once more.