...Controlling the spine makes sense to disrupt the power of a punch. But a knife doesn't need that power. Everyone who has cut himself shaving knows it doesn't take much pressure to break skin. I could cut myself shaving while someone was trying to punch or control me. I could also cut someone else.
If I have enough room to play the defang the snake nickel-and-dime-you-to-death game, it's very hard to defend against (if you can't run). I can stay at just-touch-you range and get in enough cuts to wear you down. If I'm forced to close range, I can launch a stab at central body mass. Many blocks will still end up having the blade--which reaches several inches beyond the end of the hand--either reach the body or slide along the blocking arm. In addition, footwork is a huge part of what we do.
Kenpoists hit hard, in my experience. The empty-hand FMA strikes are, as a rule, not as hard. But you can't trade a hard hit for a stab. Even Mike Tyson couldn't count on a one-punch KO.
My point is, while you train to deny me my second option as you say, for me that second option is a built-in part of the first option. It's a chess game...and in chess, the winner is the person who makes the next-to-last mistake.
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The empty-hand FMA response to a knife typically involves side-stepping and control of the weapon hand, followed by disruption of the opponent's balance, then striking such as knees to the thigh and/or a disarm. We block in a particular way (often the palusut). But, we also figure that even a very experienced FMAer is at a huge disadvantage against even an inexperienced knifer.