Maarten,
Yeah, you're right -- videos can't convey what's really going on in the bodies, and it really does have to be felt, first-hand.
I do believe that there are different expressions of hengjin, but that they likely are generated by the same basic mechanisms. The difference comes in the "tweaking" of it to get different effects, such as a rapid pulsing created by condensing of the inguinal creases and dantian, a rapid expansion of mingmen. and rapid torquing from the ground. A slow, steady execution of those same mechanisms will create a flowing but powerful expression. Then, adding variations in sinking/condensing and rising/expanding create a "three-dimensional" effect of dipping or spiraling.
Am in agreement with you about the less-than-realistic conditions that demonstrations present. However, in the videos I posted, these are teaching moments, with the teacher explaining and showing principles and concepts, and the effects they create. I have been present with my sensei when he was challenged by an experienced fighter who requested a freestyle, open exchange, and that bout lasted about a minute or less, with the fighter on the ground and submitted. He was winded and exhausted, and my teacher, a much older man, and also ill at that time, was relaxed and not the least bit out of breath. lol. In our AJJ training, full-force attacks are required for all practices, once the students have learned the techniques being drilled, and sparring is required in order to learn how to apply the internal and combative principles under duress.
But teaching is teaching, and when trying to explain and walk students through a method, it has to be broken down and shown on an assistant. The ones in these videos, while playing a part in instruction, are actually throwing committed attacks. Their force is being absorbed or repelled and redirected through their joints, to their center of mass, controlling their structure and making further attacks mechanically impossible. It can be done with or without causing pain, but when both the mechanical control and the pain are applied, I can't even begin to describe how shocky and debilitating the kuzushi (control of center of mass, upon contact) is! You don't even want to try a second punch, even if your body wasn't locked up and could throw one.
Regarding being "smacked around," As shocky as things might look, they are being done with great care and at a fraction of the speed and force of a "real life" application. Even so, they can be painful... but they are done in such a way that leaves no damage. When we joint-lock we all suck in our breath from the pain, but as soon as it's released, we're fine, and in fact the endorphins have us going home feeling great. lol.
Also, as students progress in their internal training, they learn how to neutralize strikes and locks, to absorb and redirect them, even to reverse them. We are able to protect ourselves in training, while letting our partners (and teacher) get in their practice and instruction points. The system is very martial, developed by men who worked in tough professions, but it's not sadistic, and we're not masochists. It's just very cool stuff with a fascinating history.