Mega Man ZX

If their Health Meter flashes red afterwords, then you've hit the Biometal location in the boss. If you hit the Biometal location in a boss and depending how many times you hit that spot, that will decrease the size of the Biometal Energy meter for that Biometal; but only the first time.

There are two ways to repair that Biometal fragment: either by paying Fleuve, which costs E-Crystals; or by getting a higher level victory against the boss.The bosses will start to respawn after you've beaten the 5th Pseudoroid boss; once you've beaten all 8 Pseudoroid bosses, there will be 4 that have respawned. And there's no limit to how many times they can respawn.

So depending on how you look at it, it can be either good or bad. Hitting the Biometal in a boss does more damage, but will also decrease the size of the Biometal Energy meter. And if you're going for a Level 4 Victory, hitting the Biometal in a boss is the last thing you want to do.