Stagger

Stagger is a Brewmaster ability that causes a portion of incoming physical damage to be dealt over 10 seconds rather than instantly.

For each hit taken, the damage that is stored in Stagger is recalculated and the dot refreshed. The key thing here is that Stagger by itself does not mitigate damage, but instead smooths it out. Also, Stagger by itself will not kill a Brewmaster monk. You can be at 1 health with only stagger ticking and not die, but in a raid there are a lot of things that can take that last point.

The amount of damage that is staggered is determined by the stagger percentage. Since BfA, the percentage is now determined by agility but is roughly around the 35% proportion in earlier expansions. This is doubled by keeping up Shuffle, and 15% Stagger can be temporarily added with the talent Fortifying Brew: Determination.

The Stagger DoT has three "Phases": Light Stagger, Moderate Stagger, and Heavy Stagger. Light Stagger is when the total stagger amount is less than 30% of max health. Moderate Stagger is when the total stagger amount is more than 30% of max health, but less than 60%. Finally, Heavy Stagger is when the total stagger amount is greater than 60% of max health. Stagger can still continue accumulating at this point. While the bar appears to be full at 1m HP of Stagger damage, there is no technical upper limit to the amount of Stagger, as long as the monk is able to stay alive.

As was mentioned previously, Stagger does not by itself mitigate damage. For this the monk needs to use Purifying Brew, which will remove half of the current staggered damage.