Warm starting
Seeding an iterative solver with last frame's impulses instead of starting from zero, so a stack of boxes settles instead of jittering.
Iterative contact solvers creep toward the answer one pass at a time and rarely reach it within budget. Start from zero every frame and you begin the same climb again — the result is a stack that never quite falls asleep but hums in place.
The fix is to store the impulses from the previous step and apply them straight away as a guess. That requires a persistent contact manifold: the engine must recognise the same contact points across frames to know which impulse belongs where. Box2D and Bullet both do this, and it is the single most important trick for stable stacking.