First things first, this information is intended to be used in self defence situations or in order to satisfy your sick curiosity. I do not suggest that you go out and try this on the first guy you run into, and I will not accept any responsibility for stupidity on the part of anyone who abuses this information.
It would seem that kicking an adversary below the belt would be a fairly straightforward technique, and one ideally suited to a street fight, where your objective should be to incapacitate your opponent(s) as quickly as possible and get the hell out of there before someone pulls a gun or a knife, or calls some friends over to help.
While a kick to the testicles can be highly effective in ending a fight, the majority of people do not know how to properly exercise this technique, and as a result their kicks may not be as effective as they ought to be.
Most people, when kicking below the belt, simply swing their foot straight up between the opponent's legs, making contact with the centre of the foot (the part of the foot on or just below the shoe laces) and striking both the opponent's testicles.
While such a kick is likely to give the adversary an initial shock, he will quickly realise that it does not hurt as much as he expected. Any real pain he experiences will probably come after the fight is over.
This is because the testicles, when subjected to an evenly distributed shock, will retract back without too much damage being done.
In order to effectively kick the testicles, you should have in mind the following principles:
1 - You should try to gather the force of your kick in a small striking area. If you have studied martial arts in any detail you will be aware that more damage is done from a single-knuckled strike than a punch which connects evenly with the whole of the fist. When you reduce the size of the striking area you will concentrate all the energy of the attack into a smaller area and do a great deal of damage.
2 - You should aim to connect with either the left or right testicle - not both. While such precision may be initially beyond your abilities, some practice in kicking small targets will quickly improve your proficiency in this. If you have studied a striking martial art such as karate or tae kwon do then you may well have good enough aim to strike a single testicle. The reason for striking only one side is to prevent the testicles from retracting.
3 - Follow through with your kick. Do not stop exerting force once contact with the target is made. This is done in order to catch the target between the striking area of your foot and bone in the attacker's groin, causing significant discomfort.
So, to put this into practice, Jill is walking down the street when she is confronted by a mugger. His stance is with his legs open, allowing access to the appropriate target area. Jill forcefully throws a rising kick between the opponent's legs and very slightly to the left of the centre line. She strikes with her toe in order to concentrate the entire force of the kick into a small area, and follows through until the kick is stopped by bone. Shew then quickly withdraws her foot to prevent the opponent trapping it.
Once again I would like to point out that this technique is not to be taken lightly. It can cause a severe amount of pain and should only be used in self defence.