It is applied by almost all sources of slow, for example including things like the Bane of the Trapped aura, Multishot Wind Chill or by simply hitting with the mentioned skills. It also works with every single Cold damage skill the Demon Hunter class has to offer, making it a solid choice and easy to apply whenever you use that element.
The effect produces a small visual cue above the enemy's head. Elite affixes that have a duration (e.g. Plagued, Orbiter) are usually snapshot from damage reduction mechanics such as Numbing Traps, so it needs to be applied before their cast.
This passive stacks multiplicatively with multiple Demon Hunters in a party.
Very strong defensive passive and support choice
If you need defense for your solo push, this is typically the go-to passive. And just in general, you can bring it for strong group utility even if your role is DPS, as it benefits everyone.