Because SPY expires every day, a fresh book of same-day options loads huge gamma into strikes near spot. Concentrated gamma means dealers must hedge aggressively as price moves — and that hedging feeds back on the very price that triggered it. A loop.

The loop in positive gamma

When dealers are net long gamma, the loop is stabilizing: a push up is met with dealer selling, a dip with dealer buying, and the more price moves the harder they lean against it. That self-reinforcing damping is what produces the tight, pinned, mean-reverting 0DTE tape — small ranges that keep snapping back.

The loop in negative gamma

Flip the sign and the loop reverses into an accelerant: dealers buy strength and sell weakness, so a move begets more hedging in the same direction, which begets more move — the negative-gamma spiral. The same concentrated 0DTE gamma that pins price in one regime rips it in the other.

Concentrated 0DTE gamma is an amplifier with a sign. Positive: it damps and pins. Negative: it accelerates and trends. The regime sets the sign.

Why it dominates intraday

Longer-dated gamma is spread thin across many strikes and expirations; 0DTE gamma is concentrated and enormous, so it overwhelms the intraday hedging picture, especially into the afternoon as expiry nears. That's why reading the same-day dealer map — and knowing which side of the flip you're on — is the single most useful lens on how today's SPY tape will behave.