The best way to knee someone in the groin, is when they have you by the arms and they’re trying to pull you somewhere.

Pull away

Pull away initially to make him think you’re going to engage in a tug-o-war. This lures him into a false sense of security. A contest of strength is a contest a larger, stronger attacker will feel like they can win.

Let him pull back

When he feels you pull away he will just pull back twice as hard. When he does this, go with it. Launch yourself at him and pull yourself towards him. The combined force of both of you pulling each other together will let you land a blow harder than either of you could produce alone.

Kick ‘em where it hurts

First lift your knee. Pull your leg all the way up so your foot is as close to your bottom as possible. Then drive your knee directly toward him like a battering ram as the two of you are headed towards each other. This is not an upwards blow from underneath, just hold your knee up and push it straight forward. Use your hips and put your whole lower body into it.

This is a commonly expected move but works best with the element of surprise, therefore I would recommend never doing it first unless he is wide open. Instead try to get his attention off his groin by attacking his nose or eyes first, just before you go for this move.

While you may not realize it, this move also works just fine on female attackers.

From a bearhug

If your attacker has you in a front bearhug you can lift your knee, hard and fast, into the groin from below instead of using the pulling technique described above.

While it is not as powerful from this position, it is almost impossible for him to block so you can keep landing these until he releases you.

To get a little more space to execute this version of the maneuver, you can try a groin slap first and he will move his hips away from you (and right into perfect kneeing range).