Charm measures how an option's delta changes purely with the passage of time. As expiration nears, out-of-the-money options bleed delta toward zero and in-the-money options firm toward full delta — and because dealers hedge delta, that decay forces them to trade the underlying. Into a Friday expiration, charm becomes a real flow.
What charm does into the close
As Friday afternoon wears on, the delta of the huge expiring options book shifts fast from charm alone. Dealers unwind hedges they no longer need — buying back shorts, selling longs — which creates a directional drift and, more often, a tightening pin toward the strikes with the most open interest. The market often gets quieter and stickier into the bell, magnetized to big strikes.
The Friday-afternoon “drift to a strike” isn't coincidence — it's charm forcing dealers to unwind into expiration.
How to read it
Expect pinning behavior into a big Friday close: price gravitating toward heavy strikes, ranges compressing, breakouts less likely to run. This is the mechanism behind expiration pinning and max pain. It's strongest on monthly and quarterly expirations, where the expiring open interest is largest.
How to trade it
In a charm-driven pin, fade small pushes away from the magnet strike rather than chasing them, and be skeptical of late-day breakouts — the flow is working against continuation. Keep size modest; pinned tape is choppy. And know when it ends: once the options expire, the charm flow is spent, which is part of why the days around expiration behave differently. Read it as structure, not a signal to force a trade.