On expiry days, price sometimes appears to lock onto a strike and stay there. It is not coincidence and it is not manipulation. It is the arithmetic of hedging a large position that is about to expire.
The mechanism
A dealer holding a large long option position at a strike hedges by trading the underlying against moves. As expiry approaches and the option sits near the money, the hedge ratio becomes extremely sensitive: small price moves require large hedging trades.
Those trades push against the move. Price rises slightly above the strike, hedging sells; falls slightly below, hedging buys. The result is a restoring force that strengthens as expiry approaches, because the sensitivity increases.
What makes a pin likely
Substantial open interest at a strike, price already close to it, and not much time left. All three are needed. Large positioning at a strike price is nowhere near does nothing, and a strike with little open interest exerts no force regardless of proximity.
It also requires the positioning to be concentrated. Interest spread across several nearby strikes produces a gentle zone rather than a pin.
What breaks it
Anything large enough to overwhelm the hedging flow. A macro release, a genuine order of size, a broad market move. The pin is a mechanical effect of moderate strength, not a force field, and it loses to actual supply and demand.
After the close
The positioning expires, and the force vanishes completely. Price that was held near a strike all afternoon frequently moves immediately afterwards, because the thing holding it is gone.
This is the practical point. A pin tells you something about the next few hours and nothing at all about the next session, and trading it as though the level persists is a common and expensive error.