First, it's not an option. It's in the combat section, not some mythical "optional rule" section.
The whole DMG is essentially optional rules. (really, the whole game is, but don't admit it to the players)
Specifically, it's in the section talking about hit points and damage. It explains what happens when you drop to 0 hit points. What happens? You fall unconscious.
nb: that's to /exactly/ 0 hit point. If you drop to -1 or fewer you die.
Then it explains what happens when you go past 0. What happens? You lose 1 hit point per round until -10.
That's what happens while you're unconscious, after having been reduced to exactly 0. You lose 1 hp per round, going from 0, to -1, etc, down through -9, then die when you reach -10.
If you don't take any more damage from an outside source. It's not super-clear what happens if you get hit again when at negatives from bleeding. However, judging from the optional rule, below, if you get hit, again, after reaching exactly 0, well, even if it's only 1 hp, you die.
"...you die" six letters, two little words, yet they constitute so much of old-school D&D...
...sorry, waxing nostalgic there for a moment.
The ONE AND ONLY OPTION it speaks about is that the DM can extend unconsciousness to up to -3 hit points if one blow takes you to that range.
What that actually means is that, under said option, if you are taken from a positive number to 0, -1,-2, or -3 by a single attack, you are unconscious, instead of just when being reduced to /exactly/ 0. You'll start 'bleeding' from there.
But, again, it seems if you are hit again, or, say, reduced to 0, then hit for even one more point, well, <broken record> "you die."
They're really neither of them very generous options compared to 2e Death's Door.