“Hard on soft, soft on hard” is a simple concept in Krav Maga.
If you are going to hit a hard object, or, let’s say, a hard part of the body, then you should use a soft part of yours. On the contrary, if you are going to hit a soft target, or part of the body, then you should use a hard part of your body.
This, again, is more of a guideline to help you avoid injury, another important aspect of Krav Maga.
The classic example is, if you are going to hit someone in the face, say targeting the nose, you should hit them with a closed fist, using the hard, top two, large knuckles. However, if you notice they are tucking their chin to change the target to their hard forehead, you should switch to a “soft” palm strike instead. Again, the main goal is to avoid injury more than anything. If you were to punch with the knuckles to the crown of the head, full force, it is likely you will break your knuckles, as the forehead is harder than your fist.
You could make the argument that this is similar to Tai Chi “yielding” concepts. Another argument would be that you can simply toughen your body using hardbody training techniques, as featured in classic Okinawan Karate or other similar styles. However, this takes far too long for the average Krav Maga practitioner. To properly develop hard body training, to kill the nerves on the surface and increase bone density, it takes dedication and time (often in the length of years) to get the desired result. As Krav Maga is supposed to have a quick and easy learning curve, most schools would not want to wish to take the time to do such things.
Again, the “Hard on Soft, Soft on Hard” is a guideline more than a rule, but it can save you to fight another day, by preventing injury. For defending yourself, for survival, do whatever you have to do, but, in the modern world, breaking a hand in the process could become problematic for anyone who needs their hands for work.
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