Pin risk is the tendency of an underlying to gravitate toward a heavily traded strike as expiration approaches, then stall there. For a trader holding options at that strike, it is the risk of arriving at expiration not knowing whether you will be assigned.
Why price pins
The effect comes from dealers hedging long gamma at a strike.
A dealer long gamma at $100 must sell as price rises above $100 and buy as it falls below. That is not a choice — it is what keeping the hedge flat requires. The result is a mechanical force pulling price back toward the strike from either side.
The pull strengthens dramatically into expiration. Gamma concentrates around at-the-money strikes as time runs out, so in the final hours the hedging required for a small move becomes large. A strike that exerted mild influence on Wednesday can hold price in a tight band on Friday afternoon.
What makes a pin likely
Three conditions:
Concentrated open interest. The strike needs enough contracts for the hedging flow to matter against normal volume.
No dominant catalyst. Earnings, a macro print, or index rebalancing overwhelm the effect instantly.
A market that is already close. Pinning pulls price the last dollar or two. It does not drag a stock 8% to reach a strike.
The trader's version of pin risk
For sellers, the danger is genuine and often underrated. If you are short a call that expires exactly at the money, you do not know whether you will be assigned. The holder decides after the close, and they may decide based on after-hours news you cannot see.
That leaves you facing Monday either flat or short 100 shares per contract, with a weekend of gap risk in between. Closing a short option that finishes near the strike — rather than letting it expire and hoping — costs a few cents and removes the problem entirely.
Using it
Pinning is a tendency, not a rule, and treating it as a rule is how people lose money on it. It is most useful as context for the last day or two of an expiration cycle: knowing which strike carries the gamma tells you where price is likely to find friction, and where a breakout has to overcome mechanical resistance to get going.
Pair it with max pain — they often, though not always, point at the same strike.