I don't agree that that is a primary factor in striking power.
My power has always been a strong point, and I'm very relaxed/flowy when I strike.
It's hard to explain in a short space (which is why it can be so difficult to find a good explanation).
Some guys can't keep cued the muscles they need to cue without tensing
all of their muscles at the same time; this is a neuromuscular engram issue and needs to be corrected with instruction. Guys like this when they spar hard or fight and want to hit harder will be all tensed up, and ironically reduce their power due to different muscles working against each other, will gas out due to extraneous muscle contraction. More experienced guys will often then tell these guys to relax, and if they do they will have better endurance. Still not hitting any harder yet though.
Other guys, on the other hand, can cue just the muscles in the kinetic chain they need for the movement they need, and only those muscles, while everything else is relaxed; can cue them at just the moment they need, and be relaxed before, and more quickly back to relaxed after. You can call these people 'naturals', and can potentially become even better with more conscious instruction.
In any case, if the full chain for a movement (like a punch) is not cued
at the moment of impact, if you are more lax at one or more points along the kinetic chain at the moment of impact, then your impact will be 'squishy', will have energy leaks, as if you were hitting them with a pillow or shock absorber, rather than a solid piece.