Gamma flip — why the same news gets absorbed on one day and amplified on the next

Two days, two pieces of news of about the same size. On one, the index swings twenty points and comes right back. On the next, it goes straight through to the downside.

The news alone does not explain that. What differed was the options already sitting on the market.

This piece walks through that structure from the beginning. You do not need the Greeks going in — we start with delta.

1. Delta — how much the option follows

If the underlying (say, the S&P 500) rises one point, how much does the option rise? That ratio is delta.

Delta	Meaning
0.50	The index rises 1 point, the option rises 0.5
1.00	Moves nearly one-for-one (deep in-the-money call)
0.05	Barely moves (far out-of-the-money)

Calls run 0 to +1, puts −1 to 0. Puts lose value when the index rises, so the sign flips.

Delta also reads as "how many shares am I effectively holding." An index option typically carries a 100 multiplier, so one 0.50-delta call carries roughly the directional risk of 50 shares of the index.

2. Dealers do not bet on direction

Enter the dealer — the market maker who takes the other side when we buy or sell.

Their business is not calling direction. They earn the bid-ask spread and time value. So the moment an option lands on their book, they erase the directional risk it created.

Take the example above. A dealer ends up holding one 0.50-delta call. They are now long the equivalent of 50 index shares — they are exposed. So they short the equivalent of 50 shares. Now it does not matter which way the index goes; the two sides cancel.

This is delta neutral.

You cannot trade the S&P 500 index itself the way you trade a stock, so the hedge is done in futures. The mechanism is the same.

3. Gamma — hedging once is not enough

The problem is that delta does not stay put.

As the index rises, that call moves further in-the-money and its delta grows from 0.50 to 0.60. As the index falls, it shrinks to 0.40.

How fast delta changes is gamma.

So the dealer cannot hedge once and walk away. Every time the index moves, they have to re-hedge. That repeated adjustment is dynamic hedging.

And this is where the whole market's character splits in two.

4. The fork — which way is the dealer holding gamma

① Dealer long gamma → the market gets damped

The dealer holds one call, delta 0.50, gamma 0.10. They have already shorted 50 shares' worth to sit neutral.

Index moves	New delta	Hedge needed	Dealer does
+1 point	0.60	short 60	sells 10 more
−1 point	0.40	short 40	buys 10 back

They sell into strength and buy into weakness.

The dealer trades against the move. Rallies meet selling, dips meet buying. The market stays in a narrow range, and news pops and fades.

② Dealer short gamma → the market gets pushed

Now the dealer has sold one put. A short put carries positive delta (say +0.30) and negative gamma (−0.10). They have shorted 30 shares' worth to sit neutral.

Index moves	New delta	Hedge needed	Dealer does
+1 point	+0.20	short 20	buys 10 back
−1 point	+0.40	short 40	sells 10 more

They buy into strength and sell into weakness.

Exactly the reverse. The dealer trades with the move. Up begets more up, down begets more down. Small shocks turn into large ones.

Same dealer, same discipline, opposite effect on the market. Not because dealers are helpful or harmful. Only the sign of what they hold is different.

5. Summed across the market — GEX

So far, one contract. A real market carries tens of thousands of strikes and expiries.

Multiply gamma by open interest at every strike and add it all up and you get GEX (gamma exposure), a measure SqueezeMetrics put forward in 2016.

GEX positive → dealers net long gamma → damping
GEX negative → dealers net short gamma → amplification

And GEX changes with the index level, because which strikes sit at-the-money changes as price moves.

The gamma flip is where that sum crosses zero

There is a level where GEX is positive above and negative below. That level is the gamma flip, also called the zero gamma level.

	Dealer re-hedging	Character of the day
Above the flip	sell into strength, buy into weakness	damped, mean-reverting — tight ranges, news absorbed
Below the flip	buy into strength, sell into weakness	amplified, trending — moves run in one direction

That is why identically sized news lands so differently on different days.

6. ⚠️ What has to be said plainly here

GEX is an estimate, not an observation.

Dealer positioning is not published. All anyone can see is open interest by strike. Whether the dealer side of it is long or short is not disclosed to anybody.

So GEX rests on four assumptions, stated in the original white paper:

#	Assumption	What it costs you
1	Every option trade is facilitated by a delta-hedger	Counterparties who do not hedge fall out of the math
2	Calls are sold by investors and bought by dealers (call overwriting and collars drive the call market)	In a heavy call-buying regime the sign can invert
3	Puts are bought by investors and sold by dealers (protective put demand)	Same, in a heavy put-selling regime
4	Dealers hedge exactly to the option's delta	In practice they use hedging bands and adjust in steps

Assumptions 2 and 3 are the load-bearing ones, and also the weakest. Treating call open interest as positive gamma and put open interest as negative gamma comes entirely from them — and that is an inference about market convention, not a verified fact.

So "GEX is +5B today" does not mean "dealers are long this much gamma." It means "if the assumptions above hold, it would be about this much." Erase that difference and an estimate quietly becomes a fact.

7. How I actually use it

Three things.

① I read character, not direction. The gamma flip does not say up or down. It says whether today is a day that absorbs moves or spreads them. Used as a directional signal, there is no reason for it to work.

② I judge it on closes. Brushing the line a few times intraday means little. The dealer book's character changes when price settles on the other side of it.

③ I do not memorize the number. The flip moves daily. An expiry rolls off, open interest changes, the index travels, and yesterday's level means nothing today. Yesterday 7,745, today 7,785 — a 40-point shift overnight is ordinary. It has to be recalculated every day.

8. What I do not know

Leaving this in, honestly.

Public validation statistics for the gamma flip are thin. The white paper the concept comes from relates GEX to subsequent realized volatility, but I have not yet found published work testing the specific claim "trends develop below the flip" on a large sample.
Dealer positioning is still undisclosed. I do not know when those regimes arrive — retail piling into calls, for instance — or how often.
So I use this as a description of the day's environment, not a trade signal.

This is a description of a mechanism, not a recommendation to trade any security. Your decisions and their outcomes are your own. Actual observations I record separately, with the date attached.