Delta tells you how fast an option moves; gamma tells you how fast the delta itself changes. And gamma is not spread evenly across strikes — it concentrates at-the-money and grows enormous as expiration approaches.

Why it peaks at-the-money

Think about what delta does across strikes. Deep in-the-money, delta is pinned near 1.0 and barely changes. Far out-of-the-money, it's near 0 and barely changes. It's right around the strike — at-the-money — where a small move in SPY flips the option between “likely worthless” and “likely valuable,” so delta swings the most. That maximum rate-of-change of delta is gamma, and it lives at-the-money.

Why it explodes on 0DTE

Gamma rises as time runs out, because near expiration a small move is the difference between finishing in- or out-of-the-money with no time left to recover. On a same-day at-the-money SPY option, gamma is huge: delta can rocket from 0.40 to 0.70 on a single fast push. That's the whip — your position gains delta (and dollars) explosively in your favor, and loses it just as fast against you.

ATM gamma is the engine and the hazard: the same force that makes a 0DTE winner run makes the reversal brutal.

Why it matters for the tape

This isn't just your P&L — it's the market's. Dealers hedging that concentrated at-the-money gamma is exactly what pins price to big strikes and defines the call and put walls. Understanding your own gamma and the dealers' gamma are the same lens. For the scalping implications, see how to scalp SPY options off dealer levels.