I am growing more and more convinced that the "whipping" model of punching technique I wrote about earlier this month is a dead end. Read this blog post and watch the videos to see a very clear description of whipping technique. I'm pretty sure it's a bad idea (no disrespect to anybody intended).
Briefly, in a whipping punch you rotate the hips first, relax the torso, and let a "waveform" pass through the relaxed torso, relaxed shoulder, and into the fist. This may or may not be what some refer to as "staged activation." It feels very powerful, and if you scout around on YouTube you can find some advanced karateka utilizing the technique. Look for the hips to pre-load (pull back before launching the technique, then start the technique by rotating before the hand moves) and look for the shoulders and hips to move separately (even if only by a small amount) and, most telling, look for the hip to be stopped or even pulling back when impact is made.
Look at this from a fairly simplistic physics viewpoint - is there any conceivable mechanism by which having your hip pulling back at the point of impact could actually make your strike harder? Remember, you're trying to hit your opponent. When you snap a wet towel the "pullback" doesn't make the tip of the towel hit someone harder - it makes the noise louder because a greater force is put into the towel. That's very satisfying, but it's not going to make for good technique.
Why does this "whipping" motion feel so powerful (because, in my opinion, it really does, which is part of why people fall for it)? Well, think about where the feeling of strong technique comes from, especially when you strike air. You're not feeling the impact of your fist - it's not even hitting anything. If you hit a bag or striking pad, you still don't feel the impact - the person holding the pad might, but if you feel the impact it's only in the nerves in your fist, not from the rest of your body. The sensations you get in your shoulder, core, hips, etc. are feedback from receptors in your own muscles, tendons, and ligaments that are reporting to your brain about forces generated in those areas.
When you "whip" a technique - when you pull the hip back while the technique is at full speed - and especially when you relax your core - you're going to get a tremendous amount of force through your body. Basically, you're adding the momentum of the hip pulling back to the momentum of the fist traveling forward and stopping all that momentum in a very short time. That's going to require a lot of force!
Imagine Superman has to stop an out of control train by pulling on a chain or rope that's tied to the back of the train. In one instance he braces himself, standing still, and when the slack in the chain is gone he'll feel a huge jolt and stop the train. Now imagine that Superman decides to stop the train by running in the opposite direction very fast. He grabs the chain and runs or flies in the opposite direction. Once the slack is gone he'll feel another tremendous jolt as he stops the train. Which "jolt" will feel more powerful to Superman? I'm sure the one where he's traveling backward - he'll have to stop the train and himself. Does that mean the train was going any faster? No, of course not.
By pulling your hip back, or stopping it before the "impact" (before you stop the punch), you increase the subjective feeling of force because you're making your body pull harder to stop the punch. You're making a bigger "jerk" travel through your body. Does that mean your fist was traveling faster or harder (or in any way which would cause more damage to your opponent)? No. In fact, it was probably moving slower towards the opponent - because the hip wasn't driving it forwards anymore.
If your hip is stopping at the same time as your fist, roughly, the "jerk" you feel might be less even if the punch is stronger (going forward at a greater speed). You're not snapping the towel - you're not adding the momentum of a backwards moving hip to the momentum of your punch moving forwards. You'll feel less force at the impact and deliver more force to your target.
Watch this video of Tommy Carruthers, a JKD instructor:
It's not the easiest thing to see, but he's snapping the hips through his strikes at the point of impact, not before. And he's fast. That's good movement.
In addition, by keeping the torso tight instead of relaxing and twisting to create a waveform you're preventing rotation of the spine. That's good from a health perspective - making a waveform actually requires relaxing the core and twisting the spine at the same time, which is pretty much what I'd tell someone to do if they were intentionally trying to injure their own back. The whipping or waveform motion puts extra stress on your body while intentionally making your body less prepared to withstand that force while reducing the impact delivered to the target while slowing down the technique. I can't see the good in it.
How do we train to strike properly? Practice punching my moving the fist first, then snapping the hips through at the point where the fist should be making contact with the target (when the arm is about 85% extended, give or take). Keep the torso very tight (your "core") and lock down the shoulder (tense the lats) at impact. The timing isn't easy, and it's not as satisfying as using a giant pre-load, whipping the hips around, torquing the torso, then snapping the hips back at contact. The latter feels wonderful and powerful, but all you're doing is pulling force out of your target and into the connective tissues of your own body.