If your main character is the master looter, and is also set to opt-out of loot, they will only see loot of the threshold level you have set or above.
So if the loot threshold is greens, then corpses with green or better items on them will glow, while all other corpses will not.
If you are boosting, and want maximum speed, setting your loot like this will let you loot all the greens (or blues if threshold is set to rare or higher) that drop, but you can skip most of the corpses.