A swift kick to the plums will have him cowering in pain. Used mostly for comedic effect.
The quickest way to stop human (and most humanoid) males dead in their tracks is a well-placed strike to the groin. Generally speaking, this only works on men--for obvious reasons--however, a properly applied strike to the female groin can also cause plenty of pain.
In anime & manga, groin attacks are used mostly for comedic effect. Most scenarios see a female character punishing a male character with a surprise kick, though stray objects landing in the groin are also common, particularly sports equipment. Sometimes, falling characters even land on their groin.