What happened Friday, and why didn't it look like the volatility event it should have been?
Roughly $7 trillion in options notional expired on Friday, September 18, the second-largest triple witching on record. The S&P 500 didn't sell off into the close. It rallied 1.1% to close at 7,637.76, and the VIX actually fell that day, to 14.81 from a prior close of 15.44.
Bloomberg reported the expiration covered close to a quarter of the entire market. By the size of the number alone, that should have been a violent session. It wasn't. That gap between what the headline number implies and what actually happened on the tape is exactly what gamma exposure trading is for.
What is gamma exposure, and what does it actually measure?
Gamma exposure, usually shortened to GEX, measures how much delta hedging options dealers are forced to do as the underlying price moves through different strike levels. Every time a dealer sells an option, they typically hedge the resulting exposure by buying or selling the underlying stock or index. Gamma is the rate at which that hedge has to change as price moves, so GEX is a running estimate of how much forced buying or selling is sitting on the sidelines at each price level.
It isn't a prediction of direction. It's a map of who has to trade, and in which direction, once price gets there.
Why does the size of an expiration matter less than which side of the line dealers are on?
This is the part most explainers skip: a big expiration tells you almost nothing about which way volatility is about to move. What tells you something is the sign of dealer gamma going into it, not the notional size rolling off.
When dealers are net long gamma, meaning positive GEX, their hedging works against the direction of any move. Price rises, they sell into the rally; price falls, they buy the dip. That flow dampens volatility. When dealers are net short gamma, the opposite happens: they hedge by buying into strength and selling into weakness, which amplifies whatever move is already underway. A $7 trillion expiration landing while the market sits in a positive-gamma regime can pass through quietly, with dealers unwinding hedges in a stabilizing pattern rather than a destabilizing one, which is consistent with Friday's session: a large mechanical event that resolved into a calmer VIX print, not a spike.
The lesson isn't that large expirations are harmless. It's that the notional figure everyone quotes is the least useful number in the story. The regime dealers are sitting in going into the event is the one that actually decides whether it's quiet or violent.
How do traders actually use this instead of just reading about it after the fact?
The practical version of gamma exposure trading is watching where the GEX zero line sits relative to current price, not just noting that dealers are "long" or "short" gamma in the abstract. Price sitting comfortably above the zero line into an expiration points toward the dampening scenario. Price sitting close to or below it raises the odds that the same size of expiration produces a much rougher session, because dealers are hedging in the direction that adds fuel rather than absorbing it.
OpticAlpha's GEX Analysis feature plots this directly: a chart of net dealer exposure by strike, the zero line itself, and a squeeze radar that scores each setup from 0 to 100 on flow, positioning, and momentum, so the regime question doesn't require rebuilding the calculation from raw options data by hand.
What should traders actually take from this heading into a data-light week?
With no major US economic release scheduled from Monday through Thursday next week, and the market's attention split between the Trump-Xi summit and a run of central bank speakers, there's no scheduled catalyst forcing a repricing. That's exactly the kind of environment where the gamma regime, not the headline calendar, ends up doing most of the work in whether price action stays contained or breaks loose. Checking which side of the zero line the market sits on matters more in a quiet week than a loud one, because there's less other noise competing with it.
Track live GEX by strike, the zero line, and the squeeze radar at opticalpha.net/terminal. 14-day free trial, no credit card required.