The short answer
Three separate flows land in the same window, and they get merged into one story called power hour.
The first is the closing auction. Market-on-close and limit-on-close orders accumulate all day and cross in a single print at 4:00pm Eastern. Index funds trade there because their benchmark is struck at the close, so tracking a benchmark means transacting at the same price it uses.
The second is hedge rebalancing. A desk that was delta-neutral at the open is not delta-neutral at 3pm, because the deltas of the options it holds moved while it stood still. Bringing the book back to flat is a trade in the underlying, and it is opinion-free.
The third is same-day option positions resolving. Contracts expiring today either get closed before 4pm or settle, and both paths remove the hedge attached to them.
None of the three is a forecast, and none of them tells you which direction price goes. What they do is explain why the volume distribution across a session is shaped the way it is.
The auction, and why index funds are in it
The closing auction is a single-price crossing run by the primary listing venue. NYSE and Nasdaq both begin publishing imbalance information in the final minutes of the session, which is why the tape often changes character around 3:50pm: the size of the resting imbalance becomes public before it prints.
The structural reason it is large is passive indexing. A fund tracking an index is measured against that index's closing values, so the way to avoid tracking error is to trade at the close. Rebalance days, quarterly index reconstitutions and month-end make that concentration larger still. The triple witching explainer covers the quarterly version, where index futures, index options and single-stock options all settle in the same session.
This part of the last hour has nothing to do with options positioning. It is worth separating, because a large close is frequently attributed to dealer hedging when the imbalance alone accounts for it.
The hedging piece, and how large it was on one day
An option's delta changes with the clock even when price does not move. That is charm, and the sister effect for a move in implied volatility is vanna. Vanna and charm explained covers what the two Greeks are and why the direction of the flow they produce depends entirely on positioning, and vanna exposure and charm exposure covers the aggregate figures and the units they are quoted in. This page is only about where in the day the flow lands.
Here is one session, measured. On 2026-09-22, at a capture taken at 1:33pm Eastern, the SPX option chain carried net charm of about minus 43.3 billion dollars of dealer delta per day and net vanna of about plus 61.2 billion dollars of dealer delta per implied volatility point. Holding spot and every contract's implied volatility fixed and moving only the clock, the book's dealer delta between that capture and the 4pm close drifted by about minus 3.0 billion dollars, of which about 1.2 billion came from contracts expiring the same day.
The shape of that drift is the interesting part, and it does not match the usual telling. Between 1:33pm and 3:00pm the running total reached about minus 2.1 billion. The final hour added about 0.9 billion more. That is roughly 30 percent of the remaining drift in roughly 41 percent of the remaining time, so on that day the decay was slightly front-loaded rather than piled into the close.
The reason is mechanical. Charm on a same-day contract does not keep rising into the bell. The deltas it is moving finish resolving, so the instantaneous rate falls: on the same capture, net charm ran at about minus 43.3 billion per day at 1:33pm and about minus 17.1 billion per day by 4:00pm, with the same-day component at zero by construction once those contracts have no time left.
Two limits travel with every figure above. The ramp is not a prediction, because it holds spot and implied volatility at their captured values and moves only time. And the sign on all of it rests on the convention that dealers are long calls and short puts against customer flow, which is an assumption about who holds which side of the open interest rather than an observation.
Why the window got busier than it used to be
Same-day contracts are the reason the third flow exists at all in its current size. When a large share of the open interest in an index is created and extinguished inside one session, the hedge attached to it is also created and extinguished inside that session, so the unwind has nowhere to go but the afternoon.
How same-day options behave covers the contracts themselves, and 0DTE gamma exposure covers the levels computed from the front expiration alone, which is the book that stops existing at 4pm.
What the last hour does not tell you
It does not tell you direction. Every flow above is a rebalancing requirement, and a requirement to buy exists just as readily in a falling tape as a rising one.
It does not tell you size in advance, either. The auction imbalance is published minutes before it prints, and the hedging figure is a measurement of a book as it stood at a capture, not a schedule of orders anyone committed to.
What it supports is a narrower reading: when the last hour moves and nothing happened, there is a mechanical account available before a narrative one is needed. The live SPX figures sit in the vanna and charm exposure section under the free gamma levels, dated with the capture they were computed from.
Educational content, not financial advice. See our risk disclosure.