Continuous bridge / deck watch monitoring position, swing, and chain tension to detect dragging.
In practice
Bridge maintains continuous watch; OOW takes position fixes every 15-30 minutes. Modern radar overlay marks the swing circle. Drag detection uses GPS or radar fix progression.
Regulatory detail & full definition
Continuous bridge or deck watch maintained while at anchor, monitoring position, swing, chain tension, and weather to detect dragging or fouling. STCW A-VIII/2 Part 4-2 requires the watchkeeping arrangements at anchor to be appropriate to the prevailing conditions. The OOW maintains GPS or radar position fixes, observes swing-circle clearance to other anchored vessels and shore, monitors weather forecasts, and verifies the brake is holding. Engine-readiness state varies with conditions; in heavy weather the master commonly orders the engine on standby and additional crew on the bridge.