The first Mega Man game (like hundreds of other games) has a 'feature' where if you get hit, your character flashes for a few seconds and is invulnerable. And in Mega Man, this also extends to the boss characters. Probably to keep you from trapping them in a corner and getting a cheap victory.

But you can totally get a cheap victory anyway.

If you hit a Robot Master with a shot of any weapon you like, he'll start to flash

But if you hit Select to pause the game and wait a few seconds

You'll find that the momentary invulnerability he enjoys will wear off while the game is frozen.

Allowing you to unload a stream of shots, and as long as you pause between each one, they're all going to hit home.

Making it much easier to win.