SPY gamma pin and how it resolves

Gamma is highest for options at the money and close to expiry. When dealers hold a large long gamma position at one strike, every move away from it changes their delta enough that they trade against the move to rebalance. That hedging flow is what holds price near the strike.

The effect needs dealers to be long gamma at that strike. When they are short gamma, their hedging chases the move instead, and a strike with large open interest can act as an accelerator rather than a magnet.

Most pins end at the expiration they belong to. Once those contracts settle, the hedges they required are unwound and the pull disappears, which is why SPY often moves more freely the session after a large expiry.

A pin can also break early. A news shock, a large flow that moves price past the strike, or open interest rolling to a new strike can each end it before expiry. Watch whether open interest at the strike is growing or shrinking during the session.

Look for one strike near spot that dominates gamma exposure by strike, with the total in positive gamma territory, a day or two before expiry. Max pain often sits close to the same strike, but it is a separate measure and does not cause the pin on its own.