Pendulum Swing — Design Codex

What it is. A tether pins the player to an anchor; gravity turns the leash into a pendulum. Fire the rope, ride the arc, and release at the apex to launch. The swing is passive — the skill is when you let go.

Player fantasy / why it’s fun. You are not moving the character; you are steering physics you don’t fully control. The satisfaction is a conversation with momentum — you commit to an arc, read it, and cash it out at exactly the right frame. Spider-Man, Umihara Kawase, Worms’ ninja rope: all deliver the same thrill of borrowed velocity spent well.

The verb

Attach → fall into arc → release. The player picks an anchor and a moment; the sim owns everything between.

How it feels / why it’s fun

Delayed agency. Input is sparse (attach, reel, drop) but consequence is huge. That gap is the flow. See pattern-mastery-and-flow.
Legible momentum. A good swing telegraphs its own exit velocity — the player sees where the arc will fling them before committing. This is pattern-readability applied to motion.
Risk you author yourself. Longer rope = faster apex = more distance but less control. The player sets their own stakes every swing. See pattern-risk-reward.
The apex is the reward. Nailing a release that carries into a second swing with no ground-touch is the mastery beat. Chain them and you get pattern-escalation-and-payoff.

Tuning levers

Lever	What it does	Sane default	Push it
Rope length	Arc radius; longer = wider, faster apex	4–6 player-heights	Variable (reel in/out) = the whole depth of the mechanic
Release-velocity carry	Fraction of tangent speed kept on drop	1.0 (full carry)	<1.0 punishes; >1.0 (boost) is arcadey and forgiving
Gravity	How hard the arc pulls down	Match world g	Lower g = floatier, more forgiving reads
Attach window	Range/aim assist to grab an anchor	Generous cone	Tight = precision; loose = flow-first
Reel speed	How fast rope shortens (raises apex)	Zero (fixed rope) is fine to start	Non-zero unlocks pumping and vertical gain
Air control post-release	Drift while airborne	Small nudge	Large = platformer; none = pure ballistics
Anchor cooldown	Reuse delay per anchor	None	Cooldown forces route variety

Start with a fixed-length rope and full velocity carry — that alone is a complete, teachable toy. Add reeling only once the base arc reads clean.

Why determinism is non-negotiable

The swing is a contract: the player commits to an arc on faith that the same input yields the same arc every time. If the physics wobble frame-to-frame, the release timing skill evaporates and the mechanic feels like a slot machine — see antipattern-input-lie and pattern-fairness-and-trust. Run the integrator on a fixed timestep off a deterministic clock (never wall-time), and let the player build muscle memory against a stable arc. Speedruns, ghost replays, and level proofs all depend on this. The verification bar lives in this module’s verify-with target (design/FUN.md#19-·-physics-arcade); design to it, don’t restate it.

Slots into

Genres: genre-physics-arcade (native home), genre-metroidvania (grapple-swing as a gated traversal verb), genre-precision-platformer (swing as a high-skill movement option), genre-action-adventure (Spider-Man-style city traversal), genre-racing (rope as a cornering tool).
Anchors: anchor-celeste (release-timing as expressive movement), anchor-katamari (trust-the-physics motion feel), anchor-cuphead (tight arcs against precise hazards).

Twist seams

Swing but the anchor is a live enemy that reacts (perspective): the pivot is a flying foe that jerks away, dives, or tries to shake you loose — your arc is now a negotiation with an system-enemy-ai agent, not a fixed nail. Combat and traversal fuse; see system-encounter-design.
Swing but each rope snaps after one arc (constraint): one attach = one arc, then the rope is spent. Every anchor becomes a consumable, and a room becomes a route-planning puzzle of which pivots to spend and in what order — flow bends toward system-map-and-navigation and pattern-pacing-and-tension.
Swing but you carry weight that alters the arc (resource): a payload changes mass, so the pendulum period and exit velocity shift — heavier cargo = slower, deeper arcs. Ties the traversal verb to an system-economy of what you’re hauling.

How it wires to Hayao

Model the tether as a distance constraint to an anchor point and integrate under gravity on a fixed timestep. On release, hand the body its current tangent velocity — that carry is the mechanic.
Study a single-mechanic physics lab in sandboxes/ (the physics lab) to see the integrator and constraint in isolation before wiring it into a game — that isolation is exactly what the doctrine asks for.
The rope render is pure view: draw it as a cosmetic node so it never enters the world hash — the anchor position and body state are the only things logic reads. See process-composition for the state/view split.
Anchors are level data; author them as fixed points and let a solver prove a traversable path exists before shipping the level.

Fails when…

The arc is unreadable. If the player can’t predict exit velocity, release becomes a guess — antipattern-guess-the-designer on a physics budget. Telegraph the tangent.
Physics drift. Non-deterministic integration breaks the timing contract; the skill ceiling collapses. See antipattern-input-lie.
Air control overrides the arc. Give too much post-release drift and momentum stops mattering — the swing degrades into a jump with extra steps (antipattern-false-depth).
Anchors everywhere. Blanket the level in pivots and there’s no route to read; density should be authored, not sprayed.
Free reeling with no cost. Infinite reel-in lets players cheese vertical gain and trivializes arcs — meter it or gate it.
It’s the only verb. A whole game of nothing but swinging fatigues; interleave with ground movement and let the swing be the highlight, per pattern-pacing-and-tension.