Well, it’s not like you will not use all of your strength in a fight trying to conserve energy (although you could do this is you are certainly clear you can bear your opponent using very little, this is situational), but that for example, if you are throwing a punch you could also hit with your elbow. You would be using all of the same energy that require you to hit with your fist but you are taking advantage of the already carried out movement to create two hits, alternatively, when you bring back the arm to your body, you could hit again with your elbow, and so on, and so on. This is an example, the combinations and possibilities are endless. Another key factor to take into account is that in Systema you apply as much force transfer as you can, that’s why you will see videos of people practicing it doing some weird move, you do not only hit with you fist, but with the weight of you forearm, you elbow, your should, your bust. A good example of this is the following.

Raise your arm as if you were to hit downwards, but place it like this:

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Now do the same but place your arm like this:

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The dot is only so that the head aligns with the body. With this simple adjust, you are now using gravity’s acceleration in your favour and you will hit harder even if you’d use the same amount of strength. Practice this with a partner and hit them on their shoulder, you should not try to break them but to feel the difference in both hits. For this to work, also, you should not keep you arm stiff while the trajectory is ongoing, but rather you should be flexible and soft until your fist reaches the objective, otherwise you will not be taking advantage of gravity since you will make the effort of moving your arm, instead of “letting it fall”.