Inertia

Inertia is the state in which health and shield regeneration stop while a Guardian stands still. It is applied by the Momentum modifier, which appears in Nightfall Strikes and Heroic Adventures.

Moving without sprinting does not remove Inertia. Sprinting clears it and applies the Momentum effect, which increases regeneration until the Guardian stops moving again. Regeneration delays reset on each instance of damage taken, so sprinting after a hit can skip the usual wait.