Why the Spiral Stops: The Gamma Flip

The dealer short-gamma spiral is the series' best story: a selloff forces mechanical hedging, which deepens the selloff. Part 5 turned that loop into 3.07x; Part 8 showed the number was the median of a distribution running to -32%. One thing stayed uncomfortable: in the real world, spirals stop. March 2020 didn't go to zero — it stabilized, then rebounded. This article closes the gap: V9-P5 adds the gamma flip to the kernel, and the mechanism finally explains both halves of a crash — why it deepens, and why it ends.

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The uncomfortable fact: spirals terminate

Every mechanism in this series so far has been self-amplifying. Short gamma begets selling; selling begets more short-gamma pressure; margin calls cascade. Run the loop long enough and you get -32%, -50%, worse. Markets don't do that. Even 2008 and 2020 bottomed out.

Why? Because the position that drives the spiral is not fixed. The short-gamma dealer is short because selling tail protection was profitable. When the tail actually arrives, that trade becomes violently unprofitable — implied volatility explodes, and the dealer's book flips: forced to sell on the way down, mechanically attracted to buy once the vol spike reprices the position. Short gamma becomes long gamma. The selling engine runs out of fuel.

That's the gamma flip, and it is the mechanism that turns a crash into a bottom.

The model: one evolution rule on top of the Part-5 kernel

V9-P4's honest boundary said the kernel didn't yet let gamma evolve path-by-path. gamma_flip.py adds exactly one rule:

gamma starts at -0.666 (short)                        # Part-5 estimate
once cumulative drawdown passes flip_threshold:
    gamma converges toward +0.80 (deep long gamma)     # vol spike flips the book

Everything else — the hedge_flow = -κ·γ·ret feedback, the α/β dynamics — is untouched. One rule, one new parameter family: when the flip happens (threshold), how hard it lands (target), how fast it bites (speed).

What the flip does to the single path

With the default threshold (10%):

Path	Drawdown	Amplification
No flip (Part-5 baseline)	-15.4%	3.07x
Flip at -10%	-13.8%	2.76x

The price path tells the real story — it stabilizes:

no flip:  1.00 → 0.95 → 0.92 → 0.89 → 0.87 → 0.86 → 0.85 → … → 0.846
with flip: 1.00 → 0.95 → 0.92 → 0.89 → 0.87 → 0.870 → 0.866 → 0.864 → 0.862 (flat)

Flip fires at step 3 (gamma flips -0.666 → +0.80), and the descent stops. Not a V-shaped recovery — a truncated crash. That distinction matters.

When the flip fires is everything

Sweep the threshold — i.e., ask "how much damage does the market tolerate before the flip?":

Flip threshold	Drawdown	Amplification	Flip step
-5%	-11.9%	2.39x	1
-8%	-13.0%	2.61x	2
-10%	-13.8%	2.76x	3
-15%	-15.4%	3.07x	10 (too late)
-20%+	-15.4%	3.07x	never

The pattern is brutally monotone: the later the market "self-heals," the more the damage converges to the full spiral. Below ~10% the flip materially truncates the crash; at 15% it's already too late — the damage is done before the positions flip. Thresholds, in other words, are not a detail. They are the whole game.

The distribution view (Part 8's language)

With uncertain gamma0 / shock / threshold (same priors as V9-P4, 2,000 paths):

Metric	No flip	With flip
Median	-15.5%	-14.0%
p10	-22.7%	-19.4%
p1	-28.6%	-23.7%
Worst	-33.4%	-27.2%

The flip fires in 80% of paths, and it does what Part 8 showed interventions do: it compresses the tail, not the center. Median moves ~1.5pp; p1 moves ~5pp; worst moves ~6pp. The mechanism's self-termination is a tail-shortener by construction.

The honest division of labor

This is the part I want to be careful about. The gamma flip stops the crash; it does not reverse it. In this model, price stabilizes — it does not recover. The V-shaped rebound of March 2020 came from a different engine: the Fed's QE, which this series models as intervention (flow cut). So the complete 2020 story, in mechanism terms, has two distinct acts:

1. Vol spike flips dealer gamma → the selling spiral is truncated (this module).
2. Liquidity injection reverses the flow → the stabilization becomes a recovery (the intervention modules).

Mixing those two acts into one "the market self-corrects" narrative would be wrong — and it's exactly the kind of conflation that produces bad policy conclusions ("no need to intervene, the market will heal itself"). The flip is real, and it is not a substitute for intervention. It converts an uncontrolled spiral into a controlled stabilization; whether that stabilization becomes a recovery is a separate, policy-driven question.

Boundaries

• The flip rule is a documented stylization, not a fitted model. Real flips are messier (they involve specific dealer books, vol-surface repricing, forced liquidations within the flip).
• The threshold is not calibrated to data. The sweep is the honest way to read it: this is how sensitive the tail is to the timing of the flip.
• What this module does not yet do: model the cascade inside the flip — the funds that blow up before the book turns positive. That's the next rung: short-gamma → vol spike → forced liquidation → gamma flip as the end of a cascade, not the start.

GitHub: github.com/fengyuGbt/crash_simulator — commit 1611737 adds the gamma-flip module.

The series has now covered both directions of a crash: the mechanism that makes it worse, and the mechanism that makes it stop. If you've watched a real drawdown stabilize — or the opposite — I'd like to hear what your book did at the turning point.

Currently available for freelance work — AI data automation, Python pipelines, and quantitative risk tools. Reach me at [e-mail].

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This article was written with AI assistance and reviewed by the author.

Photo by Greg Rakozy on Unsplash