Wrist locks are an important part of ninjutsu. The wrist lock, omote gyaku, is part of the kihon happo (the basic eight). It is a core technique that can be a wrist break if done with sufficient force. It can actually shear the wrist joint.
Practice extreme caution when performing omote gyaku! If you move too quickly your partner's wrist may be permanently destroyed. The locking action happens very quickly. Any abrupt movement can cause lasting damage. With that in mind I will give the steps below.
1) You and your partner will face each other. Your partner will hold their hand up with their palm facing them. You will be able to see the back of their hand.
2) The tips of your thumbs will be in the back of their hand. Place them so you can feel the little bones that run from their knuckles to their wrist.
3) You should have your fingers on their palm. You should now have a firm hold on their hand between your fingers and your thumbs.
4) Fold their wrist by moving their fingertips toward their bicep.
5) With your partner's wrist folded begin twisting their hand backwards. You do this by stepping behind them while holding the pressure on the folding and twisting action. Move so you can put their palm on the floor (behind them).
This was just the mechanical description of omote gyaku. You need to also keep your back straight. Don't bend at the waist. Make sure you also do the movements by letting your hips move you, not just your arms and hands. It will be ineffective if you try this technique on a larger opponent just using arms and legs.
The kihon happo has five techniques designed to capture the attacker. All of these techniques can be thought of as drills to learn the core principles of ninjutsu. What I have shared with you is just the basic mechanics of omote gyaku. You can learn more about omote gyaku and the kihon happo by visiting my ninjutsu training site at www.ninja-training-techniques.com.