Dealer hedging is a continuous process during the session, but the market is shut for most of the day. When price moves overnight — on futures, on news — dealers can't rebalance in real time. They arrive at the cash open with an accumulated hedging need, and that resets the map.
What changes overnight
Three forces. Price moved: an overnight gap puts spot in a different place on the gamma profile, so the flip and walls sit differently relative to price. New expirations: because SPY expires daily, yesterday's 0DTE book expired and a fresh one builds at the open. Repositioning: traders hedge and speculate overnight, changing open interest. By 9:30 the map can be materially different from the 4pm version — the point of why yesterday's flip can be wrong by 10 a.m.
The open's hedging job
On a big overnight gap, dealers open under-hedged relative to where price now is, so the first hour often carries extra flow as they catch up — which can extend or reverse the gap depending on the regime. The open isn't just a new price; it's dealers rebalancing to a map that changed while they were away.
The market sleeps; positioning doesn't. A gap is a hedging bill that comes due at 9:30 — and the levels reprice with it.
How to handle it
Don't trade the first hour off yesterday's levels. Re-read the map on the fresh open — let the new flip, walls, and regime print before committing — and treat the opening range as the market discovering the reset structure, not confirming the old one. On big-gap mornings especially, a live map beats a memorized one, because the memorized one is describing a session that's already over.