When force is voluntarily applied against anything delivered from you to a target regardless of system, there is a compression on your own body in return. Right, we've discussed this topic before. Newtons three laws of motions. The one above is the third law. The second law is the ol' F=ma or force equals mass times acceleration. The first law is essentially that an object is either at rest or in motion within an inertial reference frame unless acted upon by a force. This law is first for a reason. For instance, once a baseball is thrown at whatever speed, it cannot speed up or change direction UNLESS an external force acts upon it such as a mini rocket that kicks in halfway for speed or wind sheer for direction(3rd law). Once the baseball is thrown, that's it. This baseball also, will only go as fast as the second law distinguishes, f=ma, period(see 1st law). And lastly, this baseball will absorb as much force as it gives(3rd law) when it finally hits something or when it potentially accelerates(see 1st law).

The human body is no exception to these rules. When you throw a punch and contract your muscle, however much(2nd law), once the motion starts you can't increase that force(1st law) by contracting more after it started unless there's rebounding resistance such as contact (3rd law). Here's the kicker. Bio mechanically, you have to shut the muscle off momentarily before you can reapply force to that rebound. When you pull a muscle in your back or where ever, your body involuntarily violated this bio mechanical principle. ....Or...you can use your body to drive into the rebound while the arm muscle is in the process of trying to re-contract.

So what's all if this have to do with WC? For starters, if someone truly does a correct infamous one inch punch, they are following these laws(newton and biomechanics) and shutting their arm muscle down momentarily upon contact and reapplying force immediately into the rebound with their body. Interesting, huh?

So what's this have to do with bridging? If you notice, when a WC practitioner actually applies the "inch power" and moves someone powerfully, contact is already made and a compression has already happened. The force applied is into the rebound.

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If you understood what I wrote above, and I hope you did. WC's core function relies upon a compression of the body. There is a reason the one inch punch works the way it does. Every lineage does this compression in one way or another, even WSL/VT. So it's not that no one has 'the secret' but many don't understand how to convey it into their training. The biggest misconception IMO is that people think this function needs to somehow be "setup" like a jab to a cross in boxing. It's not there or developed for that purpose and every lineage I've seen that try to set this function up create bad habits IMO

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btw Chum kiu means "sinking" the bridge and is directly related to the function above. You 'hammer the nail'. One reason, "turning" and the balance of that turn matters.