An option is a bet on where a stock or index goes by a set date.

A new kind expires the very same day it is bought. These are called zero-day options.

They have grown so large that the dealers who sell them now move the market while hedging.

By 2025, zero-day options were about half of all S&P 500 options trading on a typical day.

Some days they topped 60% of the volume.

Average daily volume hit records, in the millions of contracts.

Small investors and big funds both piled in.

So a product measured in hours now shapes a market measured in trillions.

When you buy an option, a dealer usually takes the other side.

To avoid risk, the dealer hedges by buying or selling the underlying stock.

As the index moves, the dealer must adjust that hedge again and again.

This adjustment is called gamma hedging.

In calm times it can dampen moves, as dealers buy dips and sell rips.

In stress it can flip, forcing dealers to sell as prices fall and buy as they rise.

The first stress point is that flip. When it happens, hedging adds to a move instead of calming it.

A small drop can then feed a bigger one within a single day.

The second is speed. These options live for hours, so the hedging is fast and heavy.

The third is crowding near key levels, where many contracts cluster and hedging piles up.

The market can lurch as those levels break.

Most days, zero-day options add liquidity and let traders manage risk cheaply.

That function is real, and the market has absorbed the volume so far.

What can break is a sharp intraday move, when hedging turns from shock absorber to amplifier.

You can watch the signs. Exchanges publish options volume, and services track dealer positioning.

Sudden, sharp intraday swings near the close often carry this fingerprint.

Zero-day options are a fast machine whose hedging can quietly steer the market it tracks.

Most days it smooths trading; on stressed days it can amplify a move within hours.

The next test is the next volatility spike. Watch for sharp, late-day swings in the S&P 500.