From my perspective, the key to WCK is breaking the opponent's structure. That is the thing around which everything else revolves.

And it is the thing I look for in determining how good someone's WCK is (andin, for example, determining how well I am doing).

When we break the opponent's structure, we take away his strength and his speed, we take away his offense and his defense. If you've ever had your structure broken by someone who knows what they are doing, you feel like you are being tossed around like a rag doll. What breaking the opponent's structure provides is control (and safety).

If you don't break an opponent's structure, then he is free to use all his strength, all his speed, free to attack you -- and you have to deal with all of that.

So how can we break an opponent's structure and keep it broken? You can do that through striking him in certain ways. Is striking alone enough? Rarely. But, you can also push, pull, press, lift, jerk, etc. him, using leverage and momentum, to break and keep his structure broken.

What do you need to be able to push, pull, press, jerk,etc., to use leverage and momentum, against your opponent? You need a couple fundamental things. First, you need a solid connection, a good handle. What in WCK we call "a bridge". This isn't a momentary, fleeting connection (like a block) but one that is solid enough (the so-called "iron bridge") and which lasts long enough for you to perform the action, to lift, to press, to pull, etc. In other words, sustained contact in such a way as to provide that connection (bridge).

Second, you also need a certain body structure or way of using your body, one that not only makes it difficult for you to have your structure broken (that withstands being directly pushed, pulled, jerked, etc.) but that can you can use to push,pull, jerk, lift, press, etc. Because you don't do those things with your arm (localized muscle), you do them with your body. The bridge provides the connection, but it is the body that does the action (press with your body, pull with your body, lift with your body, etc.).

And, btw, this strategy of breaking the opponent's structure isn't unique or special to WCK -- you see it in judo (kazushi), you see it in MT clinch, you see it in wrestling, etc. How it is implemented in those various arts differs since their approaches differ.