Crypto options expire at 08:00 UTC. At that moment the expiring contracts settle in cash against a reference price, and every hedge held against them becomes unnecessary.
Those hedges are unwound. That unwind is the flow, and it is the whole of the event.
Before: pinning
Into the expiry, dealers long gamma near a large strike hedge in a way that dampens moves toward it, which is the gravity effect. The closer to expiry, the sharper the gamma and the stronger the damping — so any pinning tendency is at its most concentrated in the final hour.
At the print: the settlement is an average
The settlement price is typically computed over a window rather than taken at the instant, specifically so a single print cannot decide it — the mechanism in European style and cash settled.
Which means the level that settles your contract is not necessarily the level on the chart at 08:00, and a sharp move inside that window is partly absorbed.
After: the structure is gone
The expired open interest disappears from the book, so the gamma profile changes discontinuously. A wall that was constraining price an hour ago may simply not exist afterwards.
This is the practically useful part: a dealer map read before the expiry describes a book that is about to lose a slice of itself. On a large quarterly that slice is substantial, and the post-expiry map is a genuinely different map rather than an updated one.
Why most days it does not matter
Because the daily expiry carries a small fraction of open interest, so the unwind is small against what the market trades — the ratio problem at its most acute.
The honest rule: on an ordinary day, 08:00 UTC is a timestamp. On a quarterly it is an event, and the difference is the expiring notional rather than the clock.