The hip hinge is folding forward at the hips while keeping the spine neutral. The motor pattern under deadlifts, RDLs, kettlebell swings, and most posterior-chain work.
Mechanism — Loads the glutes and hamstrings through hip flexion-extension while sparing the lumbar spine. The pattern most people lose first — modern desk-bound movement defaults to lumbar flexion instead.
Use case — Teach with PVC dowel along spine (head-thoracic-sacrum contact). RDL is the cleanest progression. Hinge before squat in any program for a beginner with low back issues.
Caveats — Most low back injuries in the gym come from rounding instead of hinging. Don't load before the pattern is grooved.