Gamma is highest at-the-money and rises as expiration approaches, but the very last stage has a specific shape. Right before expiry, at-the-money gamma spikes to its maximum; the instant the option expires, its gamma is zero. That sharp rise-then-collapse is the gamma roll-off.

The intensifying pin

As gamma spikes into the final hours, dealer hedging per point of price movement is at its most intense (the 3pm gamma effect). Around a heavily-owned strike, that means a very strong pin — price magnetized hard to the level, ranges compressing into the close. The clench tightens precisely because gamma is peaking.

The release at the bell

Then the options expire and their gamma is gone in an instant. The hedging program that was pinning price simply stops — there's no more gamma to hedge. That's why the market can feel stuck into a big expiration and free right after: the pin didn't “break,” the gamma behind it rolled off.

Peak gamma is peak pin. Zero gamma is no pin. Expiration is the switch between them, and it flips in a single moment.

How to trade around it

Into expiration of a big strike, respect the intensifying pin: fade small pushes off the magnet, distrust late breakouts. Anticipate the roll-off: once the pinning gamma expires, the level that was holding price loses its grip, so don't assume the pin persists into the next session. On daily-expiry SPY this roll-off happens every afternoon on the 0DTE book — a smaller, daily version of the monthly expiration rhythm.