Same-day expiry options concentrate all of their gamma into one session, so dealer hedging is compressed rather than spread across weeks. When dealers are long gamma at a heavy strike, that hedging pulls price toward it into the close. When they are short gamma, the same compression produces sharp afternoon trends instead.
If you have ever watched ES grind around a round number all afternoon with no news and no reason, this is usually the reason.
Why same-day expiry behaves differently
An option a month from expiry has its gamma spread thinly across a wide range of prices. Dealers hedge gradually and the flow is diffuse.
An option expiring today is different. Its gamma is enormous near the strike and almost nothing away from it, because a small move decides whether it finishes worth something or worth nothing.
So as expiry approaches, hedging requirements become extremely sensitive to small moves near heavy strikes, and completely insensitive a few points away. That concentration is the whole phenomenon.
This has become far more significant than it used to be. SpotGamma reported 0DTE at around 59 percent of SPX options volume in 2025. Whatever you think of the trend, a majority of index options volume now expires the same day, which means a large share of gamma sits in the current session rather than out in the future.
How a pin forms
Picture a heavy strike with dealers net long gamma there.
Price drifts above the strike. Dealers sell to stay hedged, and the selling pushes price back down. Price drifts below. Dealers buy, and the buying pushes it back up.
The effect intensifies as the close approaches, because gamma sharpens as time runs out. What starts as gentle drag becomes something that visibly holds price in a narrow band.
On the chart it looks like a market that has run out of ideas. Small ranges, low volume, repeated tests of the same handful of prices. Traders describe these afternoons as dead. They are not dead, they are being actively held.
An ES afternoon session pinned to a large 0DTE strike: price oscillating in a tight band around one level from roughly 2pm to the close. Mark the strike as a horizontal line and note the contracting range into the final hour.
What it means for how you trade the afternoon
Recognising a pin early changes the afternoon completely.
Stop expecting range expansion. Breakout attempts near a pinning strike fail repeatedly, and each failure looks like the market is about to break the other way. It is not, it is being pulled back.
Fades get better, not worse. Moves away from the pin strike tend to revert. If you already trade value area rotation, a pin is a mechanical reason to expect your setup to keep working.
Targets should shrink. Asking for a large move out of a pinned market means asking price to overcome the hedging holding it. Reduce expectations rather than waiting for a move that is being actively prevented.
And watch the clock. A pin is a same-day phenomenon. Once those options expire the hedging demand disappears, which is why markets sometimes move immediately after a close that was flat all afternoon. The thing holding it stopped existing.
When the pin breaks
Pins are strong until they are not, and understanding the failure mode matters more than understanding the pin.
A pin holds while dealers are long gamma at that strike. Two things break it.
A large enough move. Push price far enough from the strike and the gamma concentration falls away, because gamma is only large near the strike. Once price is meaningfully outside that zone, the pulling force weakens rapidly and the market is free.
A change of sign. If positioning is such that dealers are short gamma at the relevant strikes instead, the identical concentration produces the opposite effect. Rather than being pulled back toward the strike, price gets pushed away from it, hard. This is the mechanic behind those afternoons where ES goes nowhere until 2:30 and then trends relentlessly into the close.
Which is the more important half of this topic. 0DTE concentration does not mean quiet. It means whatever regime you are in gets more extreme. Long gamma produces a tighter pin. Short gamma produces a sharper trend. Both come from the same compression, and the sign decides which one you get, which is exactly what GEX is telling you.
The mistake to avoid
Treating a heavy 0DTE strike as a level to trade against on its own.
A strike with large open interest is where hedging is concentrated. It is not automatic support or resistance, and whether it attracts or repels price depends on a sign you are estimating rather than observing.
Traders who learn about pinning and start shorting every touch of a call-heavy strike discover the negative gamma version of the same day eventually, usually expensively.
The level tells you where to pay attention. What the executed orders do when price arrives tells you what is actually happening, which is the subject of combining options levels with order flow.
Fitting it into the day
I note the largest same-day concentrations pre-market alongside everything else, adjusted for the ES basis.
Then it becomes an expectation to test rather than a plan. If price is drifting toward a heavy strike in a positive gamma regime and the range is contracting, a pin is forming and I trade accordingly, smaller targets, more willing to fade. If price is nowhere near it, it is irrelevant that day.
The single most useful habit here is simply asking, when a market goes quiet in the afternoon, whether it is quiet because nothing is happening or quiet because something is holding it. Those two look identical and behave completely differently the moment the holding stops.
Frequently asked questions
What are 0DTE options?
Options expiring the same day they are traded. On SPX they are available every session, and they have grown to represent a majority of index options volume, reported at around 59 percent of SPX volume in 2025.
What is options pinning?
The tendency for price to be drawn toward a strike with heavy open interest as expiry approaches. It happens because dealers long gamma at that strike sell above it and buy below it to stay hedged, mechanically pulling price back toward it.
Does pinning always happen at big strikes?
No. It requires dealers to be net long gamma at that strike. If they are short gamma, the same concentration pushes price away rather than pulling it in, producing sharp trends instead of a pin. The sign of the positioning decides which behaviour you get.
Why does ES move after the close on a flat day?
Because the hedging holding it in place expires. Once same-day options settle, the demand that was pulling price toward the strike disappears, and the market is free to move on whatever was already there underneath.
Can I trade a pin directly?
You can trade with it rather than against it, by fading moves away from the strike and reducing targets. What does not work is treating the strike as guaranteed support or resistance, because whether it attracts or repels depends on positioning you are estimating rather than observing.
Do 0DTE options make markets more volatile?
They concentrate gamma into the current session, which makes the existing regime more extreme rather than adding volatility on its own. In a long gamma environment they tighten the range. In a short gamma one they sharpen the trend.