I dont tend to deliberately tense my fists on the moment of impact but i agree with Rob that there is, and has to be, some tension to even hold a fist or align properly.
This is an interesting f-in question. I think the words relaxed and tensed are unclear because they're relative. Nobody can possibly mean relaxed like a noodle, or tense like uncooked spaghetti.
I have hit people barehanded and have broken some knuckles. I learned that it was important not to tense my hand up too much at the point of impact, especially if it was the head. There was absolutely no necessity or reason to tense the hand up much at all beforehand. However, in competitions where gloves and maybe wraps, the hand was already in a clenched position. But, I still clenched more when punching. Anyway, fwiw.
Afa the other fist shapes, I have to think that all of them require some amount of tension. Now, I think that spearing with the hand are the closest techniques where the hand can be almost completely relaxed and rely totally on structure.
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