Vanna: volatility moves delta
Vanna is how much an option's delta changes when implied volatility changes. For an out-of-the-money call, lower volatility means lower delta: the option looks less likely to reach its strike.
Example: a stock trades at $100 and a call with a $110 strike has three months left. At 30% implied volatility its delta is about 0.29. At 20% it is about 0.18 (Black-Scholes, with zero rates and dividends). Per contract, that is 28.8 shares of delta falling to 18.3.
Now look at both sides. A dealer short that call holds about 29 shares of stock as a hedge; after volatility falls it needs only about 18, so it sells about 10. A dealer long that call is short about 29 shares as its hedge; it needs to be short only about 18, so it buys about 10 back. The same volatility change produces opposite trades depending on the position sign.
Charm: time moves delta
Charm is how much delta changes as time passes, with price and volatility unchanged. As expiry nears, an out-of-the-money option's delta drifts toward zero and an in-the-money option's delta drifts toward 1 (or −1 for a put).
Example: keep the stock at $100 and volatility at 30%. The $110 call's delta is about 0.29 with three months left, about 0.15 with one month left, and about 0.01 with one week left. Whoever hedges that call has to shrink the hedge as the weeks pass, and the direction of that trade depends on whether they are long or short the call.
Reading them together
Vanna and charm explain why hedges can shift on a quiet day with little price movement. To estimate the flow you need the signed book, the volatility change at each strike and expiry, and the time that has passed. The VIX index summarizes S&P 500 volatility, but individual options' volatility can move differently.
Caveat: these are mechanical sensitivities, not a schedule. They do not by themselves produce a rally whenever volatility falls, a particular weekday pattern, or a predictable move at the open.