I just came up with an exercise to demonstrate a subtlety in applying certain joint locks, in particular a wing lock. The wing lock is a variation on a figure-4 (particularly evident when both arms are used to secure the hold) except whereas the figure-4 bends the opponent's arm back over his shoulder, in the wing lock the opponent's arm is bent up behind his back. It's called a wing lock because it resembles folding a chicken wing so it stays in place during cooking.
First, the setup for the lock:
The way to execute this lock is using the opposing arm to control the opponent. In other words, use your left arm to control his right, or your right arm to control his left; a mirror approach. Most people do this wrong, reaching across the centerline to grab right with right or left with left. This is both ineffective and leaves the lead side of the body exposed to counters. Thus if you enter from a forward position (face to face), simply slide your closest hand between the opponent's arm and body.
Once you enter, the focus of the wing lock is on the elbow. A common mistake is to slide the hand up the arm, putting pressure on the shoulder. This loses leverage and makes it easier for the opponent to resist and counter. As your hand slides under the opponent's arm, you should concentrate on feeling the crease of the inner elbow with your thumb. You can practice this right now by bending your right arm slightly and then sliding the back of your left hand up the inside of your right forearm.
If you lift slightly as you snake your thumb around the elbow towards the tricep tendon nerve (the slight hollow spot an inch or two above the elbow) you will feel the control. Keep focused in this area rather than reaching with your fingers towards the shoulder. You should also notice as you do this that your left elbow will be lifting the right hand. This is good, because it secures the hold. On an opponent, that allows your upper arm to press his hand towards his spine; this is where the pressure is applied to his shoulder while securing his centerline.
One can simply snake the elbow with the thumb, lifting the opponent's arm to get it to bend, and then press the hand to the spine with your upper arm. That works in an expedient manner. However, Angel Cabales sometimes demonstrated a subtle yet powerful twist of his forearm during this maneuver, using the length of his forearm as a worm gear to force the rotation of the opponent's arm. This is what my new exercise is designed to demonstrate.
Grasp the back of your right wrist with your left hand. Next hook your right hand over the back of the left wrist, a typical joint maneuver to counter the grab. Now if you just press down with the blade of your right hand, there is little effect. If you press while extending your fingers downward, there is a slight increase in pressure, but still not a lot. However, if you point your fingers forward and reach to touch your left bicep while turning the palm downward (pronate) you should notice how the rotation of your right forearm powerfully (and painfully) applies pressure to your left wrist while requiring relatively little force.
This longitudinal torque is probably familiar to many jujitsu-ists, particularly small circle adepts. A Wing Chun person might recognize this as tan sao/bong sau, using a bit of forward grinding or extension as though blocking low rather than an elbow lift for a higher defense.
There are other applications for this forearm torque, most notably for a snaking lock against an opponent's kick. Again the left vs. right (and vice versa) applies, snaking from inside the leg, going under and around. The twist/extension rotates the opponent's leg, which turns his body. As with the wing lock above, this allows you to get to his backside, a position affording you the advantage.
Developing a feel for this technique has several benefits. First, in the wing lock it can allow you to gain control using only one arm, keeping your other hand free. Second, it develops your sticky-hand energy, which increases your awareness and ability to respond to an opponent by touch.
Just remember when working with training partners that joint locks can cause injury. Go slowly enough that your partner can tap out in time, and when the locks are practiced on your, do not resist with force, as that will encourage your partner to begin wrenching harder on you in response. The key to learning locks is communication and observation. When applying locks to the arms, you'll see your control when your partner's shoulder moves. It can be subtle, but once you've taken up the slack you have all the control you'll need.