Different styles have different ideas on a tense or relaxed fist on impact. Tadashi Yamishita used to do a demo where he smashed the hell out of a lot of stacks of wood, going from one to the next, lickety split. Then he would turn to the audience and open the fist he was breaking with.
A small bird, a Japanese White Eye, which weighs a few grams, would fly out of his hand up into the rafters.
I got a chance to spend a couple days with him in the nineties and asked him about it. He only uses a relaxed fist, fingers barely touching his palm, wrist straight of course. He would hit you in the chest and it felt like a hammer. A big hammer. I've been pretty much doing it that way since. So far, no problems.
Tight fist, loose fist, it's all good if you train properly, I guess.