Below Is An Article From ehow.com
The Testicle Twist, Pull, and Squeeze Tactic:
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Grab the attacker's testicles. If you can't determine where exactly his testicles are located either because it's dark or because he is wearing thick pants, grab in his general crotch area. Executing the technique on his **** will still be effective, although grabbing the testicles will bring him much more prolonged pain.
Once you have grabbed his testicles (or whatever you can get a hold of in his crotch area), twist them in a quick, concise motion. Twisting a full turn is not necessary and will be difficult for you to do. Instead, twist a half-turn or even a quarter-turn. The key here is a fast, powerful twist that will surprise him. If you are unable to twist for whatever reason, a squeeze will be just as sufficient.
After twisting, quickly pull what you're holding. Pull as if you are trying to pull his testicles out of his body--the harder you pull, the better. Your life is in danger and you are completely justified in causing him incredible pain. Show no mercy.
Follow these steps and you will have successfully executed the grab twist pull testicles self-defense technique. The entire process should last no longer than a couple of seconds and your efforts should immobolize your attacker. Beware of grabbing, twisting, or pulling for too long or the attacker may cause you even more harm in order to stop the pain. Remember: execute the technique quickly and escape while he is trying to recover.
HERE IS WHAT THE WEBSITE http://womensselfdefense.info/120.html has to say on this subject:
This is a great move because it can be done from almost any position. It is especially useful in a bearhug when your arms are pinned within his (his mistake, since that's exactly where you need them to be!)
* Grab a handful of groin (grab with all 5 fingers, don't just pinch with a finger and thumb!).
* Squeeze as hard as you possibly can. Really dig in, and feel free to use your nails.
* At this point you may also twist, and/or pull
* If it's working, don't let go till you are sure you are safe!
The goal here is to grab at least one of his testicles (or both). If you feel that at any point you have failed you may need to readjust your grip.
If it doesn't seem to be working you can try something else instead. No sense hanging on if it's not doing anything.
IMPORTANT: If it's working, don't let go till you are sure he is no longer a threat! There have been cases where a "victim" has called the police (or even made the attacker) and waited for them to arrive before letting go!
Feel free to also strike him with your other hand, kick, bite, choke, or even headbutt!
Both of these articles are very good information for any women to remember when a man attempts to rape her. But a point of note from the first article from ehow, they say only to squeeze the man's testicles just for a little bit because if you squeeze too long the man would get too angry, but it has been my experience, in the two times I have used this technique, that once the squeeze is applied, the man is basically helpless. Sure, he may try to pull your hand away from his groin, but if he does he is only PULLING on his own scrotum, thus injuring himself more. Also, if the man tries to hit you, while you are squeezing the life out of his bits, he won't have any strength to hurt you as his entire body will be focusing on the pain between his legs.
The important thing I have read is make sure you are close to the man when trying this tactic. Also, don't just reach your hand between his legs and try to grab his testicles (unless he's naked, then it's easy to just grab those things), because it may take you a second to fish around for where exactly his testicles are, and if you give a man that much warning about what you are going to do, then he will be able to stop you. Either do A) fondle his groin area, rub it, make him feel good, get him to drop his defenses, then once you are certain of the location of his testicles, then squeeze his baby makers. Or B) slap your hand, open palm, as hard as you can into his groin area, this will stun the man for at least a couple seconds, allowing you time to get your fingers wrapped around his testicles, and then it is OVER for him as you squeeze, twist, and pull on them as hard as you can.