A mechanism exposes its cause before or with its first consequential action: a moving part, gauge, lit path, sound with a visual directional equivalent, or visible trigger volume. Quiet looking does not activate it unless it is visibly a gaze sensor. Capturing, changing a control, rewiring a station mechanism, cutting, or testing happens in world time. The journal freezes mechanisms, enemies, oxygen, projectiles, readiness, and action progress; it advances no world-time work or timing window. Compatible links between accessible mounted rig parts can be changed during that pause under [Build Rules](/Hold%20The%20Flood/Build%20Rules); physical fitting and removal still take running time.

**Learning and state.** Plate order and the plumb cage let the player predict which side will descend in a later chamber. On alive withdrawal or death, an unpinned chamber returns to neutral; deliberately placed loose objects remain. On re-entry, any remaining object above the visible load mark lights its plate and starts the normal warning rather than being ignored. Each completed release and pin persists, even if the room later has different neighbours; an unfinished release resets. The pin does not bind the room.

**Scene and read.** Two destination lamps, a floor outline, and a shutter preview illuminate before the relay commits. The floor frame accepts the astronaut, a creature, or a loose object above its visible load mark; the wrist reader classifies a recognised patient token, while every unrecognised crossing follows the previewed untrusted route. A qualifying crossing closes the previewed branch, opens the other, and sounds a local alert whose connected entrance is shown. The alert moves only eligible existing groups.

**Responses.** Observe the preview and choose which side to stand on; send an eligible loose object or enemy across the frame from cover; or complete the physical override. Its first checkpoint cuts the exposed alarm lead, preventing new calls but not dismissing groups already moving. Its second hand-cycle engages a mechanical dog in the chosen shutter track, holding that shutter open. A visibly compatible service socket may instead accept a Fork Relay to keep both local open-hold outputs supplied where their shown thresholds accept its split, but only at an authored instance and at the cost of leaving the Fork installed. No part name decodes the destination marks.

**Learning and state.** Repeated lamp order can support a later prediction about which shutter a matching relay will close; disagreement remains a hypothesis. On withdrawal or death, frame occupancy clears, the reader rearms, and an undogged shutter returns to its shown neutral state. A cut alarm lead stays cut, so rearmed detection cannot call; a dogged shutter stays open through death, while the reader may still preview and cycle the other branch. Releasing or moving the dog requires a separate completed hand-cycle. A Fork-powered state lasts only while the part remains installed and the fixture powered; installation persists, power need not. The vestibule gains no fixed post-death neighbour.

A living return preserves the arrangement, defeated creatures and spent releases; surviving groups may still move along readable routes. Death shifts unconnected geography and resets transient crowds under [Combat Rules](/Hold%20The%20Flood/Combat%20Rules). Clearing a crowd never causes a compensating elite or extra release. An arriving group uses a readable physical route, with perception, a Caller or a mechanism alert able to wake an announced source. The individual mechanism timing resets above remain separate from population persistence.

**Pressure gallery: overpower or understand.** In Ascension Works, pressers approach through a Phase-Balance chamber while a Breath Bank throws force-bearing debris across the control. Under a replaceable later **8 A** allowance, Threat Sensor → Pulse Projector supplies ordinary automatic fire, while Manual Trigger → Barrier Plate is the faced special; Dynamo is clamped to the Barrier, Capacitor stores qualifying impact charge, and its built-in manual release feeds Arc Emitter. The player faces the announced draw lane, catches debris and strikes, lets automatic fire thin the nearest valid targets, then manually releases Arc into the packed survivors. If the build clears them before the next cycle, it wins that combat cleanly: no new wave or reduced reward appears. The bank and balance chamber remain active.

**Sterile ward: create the angle or take the long way.** A Shutterback shelters pressers beside a Sterilization Carousel. A later **5 A** rig uses Threat Sensor → Pulse Projector for ordinary automatic fire and Manual Trigger → Vector Nozzle as a faced special. The authored bay shows enough clear space for the Nozzle's opposite recoil; without that clearance this response is visibly unsafe. The player waits for the shown cool detent, manually shoves Shutterback sideways, and exposes the crowd; automatic fire now has valid targets, while the next announced sweep catches bodies left in its arc. A strong execution can empty the ward without replacement pressure.

**Current coverage.** This article supplies five mechanism-family rules, a four-region pressure map and two encounter examples, including their persistence rules. [Encounter And Mechanism Placement Inventory](/Hold%20The%20Flood/Encounter%20And%20Mechanism%20Placement%20Inventory) supplies the current first-pass **13 major placed mechanisms**, including core isolation.

**Remaining design work.** Extend those briefs through exact geometry, warning and stage variants, state transitions, alternatives, current-region placement, withdrawal boundaries, persistent-mass rearming and safe or unsafe Nozzle-recoil clearance. [Production Scope](/Hold%20The%20Flood/Production%20Scope) owns the current planning estimates and shared content accounting.