Needless to say getting cornered is not an ideal self defense situation. However there are a number of ways to turn the tables on your attacker.
As you saw Ray Ellingsen explain in the video above, using a joint lock can be a very effective technique in this type of situation.
The key to this fighting move is using an effective distraction technique. No attacker is going to just let you put him into a joint lock. The open palm strike is crucial for this technique to work.
Remember though, the open palm strike is one of many possible distraction techniques for this kind of situation. You can also use a knee to the groin, a head-butt, or even spit in his face.
In fact, I would probably recommend using something other than the open palm strike simply because it will allow you to trap his hand right as you distract him (lowering the risk that he will release his hand or step back).
Another thing to keep in mind is that this self defense technique will only work if your attacker is holding onto you.
This is where the trapping technique comes in. Holding his hand against your body is essential if you want this move to work. The best thing to do his distract and trap his hand simultaneously.
After you have applied the joint lock and taken your attacker to the ground there are many self defense techniques you can use (like dropping a knee into his groin) to end the fight, but most importantly you will no longer be cornered and you have created a clear escape route.