Effect
Numbing Traps
Numbing Traps is a defensive passive that makes struck enemies deal noticeably less damage for a short time. The effect triggers from a whole range of sources – slows, chill, or certain skills like Spike Trap, Sentry or Grenades – so in many builds you maintain it almost by accident. Since the Demon Hunter is fragile by nature, weakening attacking foes' damage is a valuable survival piece.
The slot is especially worthwhile in pack and Sentry builds, where you constantly trigger slowing or trap-based effects anyway and keep large numbers of enemies suppressed. In builds without such triggers the passive does nothing. So you pick it when your setup already brings the required effects and you want tangible passive defense at no extra cost.
Which triggers count exactly
The description names two kinds of triggers. First, conditions: any enemy you Slow or Chill deals 25% less damage. Second, skills whose hits apply the effect directly – Fan of Knives, Spike Trap, Caltrops, Grenades and your Sentry fire. The weakening lasts 5 seconds each time.
The effect works per enemy. You do not need to catch every foe on screen, mainly those that are attacking you right now. You can pick it from level 30 onward.
Why Cull the Weak fits well
Among the frequent partners is Cull the Weak, a passive that also keys off Slowed and Chilled enemies. Both therefore share the same prerequisite: if you are slowing enemies anyway, a single source gives you damage reduction and extra damage at once. Caltrops is an obvious tool for this, as the skill is named as a trigger itself and also slows enemies.
Where it sits next to your other defences
Numbing Traps reduces damage at the source, while Smoke Screen and Shadow Power work on your side of the equation. These tools complement rather than overlap each other. Vengeance and Ambush often round out the offensive side, with Evasive Fire joining as the primary skill.