My point was, that any flashback didn't fit (no matter the length), since it was right between the punch, which killed the impact (like you said). There should have been a moment, where the flashback would have fit, a simple pause between his jump into the attack and the actual next punch, would have been better.
But it still feels unnecessary, since it didn't answer any question, which needed to be answered at that moment for the reader. It just misses the right dramatic background to make it work.
I disagree. It was certainly necessary, not in the right place, but it was a good and satisfatory flashback to me.