In the bad old days of Taijiquan, there was hardly any detailed teaching beyond forms correction and simple push-hands. Now-a-days, there is tons of teaching and it's getting more open all the time. What constitutes the body of taijiquan basic knowledge is really growing.
So I expect most people have heard this simple description of jin:
Peng: Twist outward, close the joints, qi rising.
Ji: Twist outward, open the joints, qi rising.
Lu: Twist inward, close the joints, qi sinking.
An: Twist inward, open the joints, qi sinking.
This is the internal structure of Taijiquan, it is one of many steps used to unify the whole body. It is the way you transform your external body's shape so that you can issue power in many different ways. It is not the source of power.
If these "mechanics" are still murky for you or your student, I suggest trying peng to the back, with your arms behind you. For it to work your peng will become: Twist inwards, close the joints and bring qi up from the heels (instead of the "bubbling well.")
Now, many people have pointed out that the opening move in many styles of Taijiquan, where both the wrists rise up in front, is usually called peng and is done with the whole body rising. This is because the jin, the method for issuing power, is left in a potential state called shi. The term shi, has a huge number of meanings including: a taut bowstring, a trigger, and strategic advantage. This can not be taught by watching, it must be felt.