SPX options are European style and settle in cash, so there is no early assignment and no share delivery. Dealers hedge with futures rather than the underlying basket, which means SPX gamma pressure expresses itself through ES rather than through individual index members.
Why 0DTE concentrates the effect
SPX expires every trading day. Same-day options carry enormous gamma per contract near the money, so dealer hedging in the final hours can dominate the tape regardless of what longer-dated positioning looks like.
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