From an option buyer's seat, gamma peaks at-the-money and explodes near expiration. From the dealer's seat, that same fact has a market-moving consequence: on a same-day chain, the dealers' gamma exposure is concentrated right around spot, and hedging it is what draws price to the strike.
The dealer's problem
Dealers hold the other side of all those at-the-money 0DTE options, and near expiration a small move in SPY flips those options' deltas violently — that's gamma. To stay delta-neutral, dealers must trade the underlying constantly as price wiggles around the strike. When they're long that gamma, they sell just above the strike and buy just below it — over and over.
Why that pins price
That repeated “sell above, buy below” around the at-the-money strike is, mechanically, a pin: dealer hedging actively pushes price back toward the strike every time it strays. The heavier the concentration at that strike (a big wall), the stronger the magnet. It's not a conspiracy to pin price — it's the unavoidable by-product of hedging concentrated gamma.
The 0DTE pin isn't intent — it's dealers forced to sell above the strike and buy below it, all day, just to stay hedged.
What it means for you
This is why same-day SPY so often gravitates to big at-the-money strikes and round numbers, especially into the close. Trade with the pin in positive gamma — fade the edges of the pinned range — and remember the flip side: when dealers are short that gamma, the same concentration accelerates moves instead. The at-the-money concentration is the engine; the regime decides whether it pins or rips.