Don't bother kicking a guy in the crotch. A straight-on kick is not going to be very effective against an attacker hopped up on adrenaline and testosterone.
What you want to do is knee him from underneath, crushing the testicles against the body. This is an extemely close-in technique; if you try to deliver it from too far away, you telegraph, giving your opponent the opportunity to guard against the incoming knee. To deliver a fast, powerful technique, you must remain relaxed; tense your muscles only at the end of the technique.
Do not expect to slay your opponent with a single technique. KEEP KICKING AND STRIKING until your opponent is laying on the ground with all the fight beaten out of him. Only then do you stop.
Xie xie, di di.