When driving a doubles combination on a slippery downgrade, what braking technique reduces jackknife risk most?

Progressive braking maintains wheel rotation on all axles, preventing the lockup that triggers jackknife. On downgrades, engine braking (selecting a lower gear before the descent) reduces service brake reliance — the engine's compression does the primary retarding work, leaving service brakes for final speed control. The trailer hand valve (which applies only trailer brakes) is dangerous on slippery surfaces as it can lock the trailer wheels while tractor wheels continue rolling — a guaranteed jackknife setup.