Vanna — hedging driven by volatility
Vanna measures delta's sensitivity to volatility. When implied volatility changes, dealer delta changes with it, forcing rehedging even though the underlying itself hasn't moved. When VIX falls — a vol crush — out-of-the-money call deltas rise, dealers buy to stay hedged, and the market gets a rally with no apparent catalyst.
When VIX spikes instead, out-of-the-money put deltas rise, dealers sell, and selloffs accelerate on their own. Vanna can dominate gamma entirely when volatility moves sharply — roughly, when the day's VIX range exceeds three to four points.
Charm — hedging driven by the calendar
Charm measures delta's decay over time. As each day passes, option deltas drift even with price unchanged, so dealers rebalance at every open — an effect that grows largest as expiration nears, producing systematic pressure at the open in the final sessions, sometimes called the opening flush.
The effect compounds heading into a big expiration: every session between now and expiry carries a little more Charm-driven rebalancing than the last, until the final days see the largest and most reliable time-decay flow of the whole cycle — the opening flush is really just Charm at its most concentrated.
The weekly cycle
Monday typically carries low Vanna and Charm activity. Tuesday and Wednesday are where Vanna is most active, moving on VIX itself.
Thursday is where Charm starts to accelerate, and the early direction of the session often reverses later in the day. Friday, expiration day, carries the maximum Charm effect at the open alongside extreme at-the-money pin risk. Reading the week means watching VIX direction and size for Vanna, and checking Charm exposure ahead of Thursday and Friday opens for the sign of the early pressure.