Out of necessity I have used the groin kick a number of time"s with great success. We train two ways, one with the ball of the foot, with a hard snapping motion. The other with the heel of the foot, with the toes pointing up. The heel kick is done with a hard thrusting motion.
I have found many parts of the body to be unreliable or unresponsive to pain. In my mind it ain't over until it's over, and until then I will use whatever I can to accomplish my objective "win". Some targets like the knee will cause much damage, as seen in many football games, and is without question a great target area to hit. The groin as a single hit will allow the aggressor time to recover if not placed well, so the secret is to never rely on a one technique strategy, but to capitalize on the reaction accomplished by any strike.
Side note, I have seen people go down while wearing a groin protector, and in Okinawa seen sensei take full hard shots to the groin area with no effect. "Not a parlor trick, but for real".