Momentum

Momentum is a modifier shared by Nightfall Strikes and Heroic Adventures that ties health and shield regeneration to movement.

Standing still puts you in an Inertia state, which stops regeneration. Sprinting removes Inertia and applies Momentum, which speeds up regeneration until you stop again. Walking without sprinting does not remove Inertia. Starting Momentum by sprinting restarts regeneration immediately, which can skip the usual delay after taking damage.