Vanna is a second-order greek: it measures how an option's delta changes as implied volatility changes. Because dealers hedge delta, vanna links their hedging directly to moves in volatility — and that link produces one of the market's quietest, most persistent bids.

The mechanism

Dealers are typically short downside puts (they sold protection). When VIX falls, the delta of those short puts shrinks, which leaves dealers under-hedged — so to rebalance, they buy the underlying. Falling volatility mechanically forces dealer buying, and that buying lifts SPY. No catalyst required; it's the hedging math working itself out as fear drains from the tape.

A vanna rally isn't optimism — it's dealers buying because volatility fell. The grind up is a hedging by-product, not a verdict on the news.

When to expect it

Vanna rallies show up most after a volatility spike fades — the calm after a scare, a Monday after an event, or the drift up as an oversold VIX mean-reverts. The tell is a slow, orderly grind higher on unremarkable news while VIX bleeds lower. It often pairs with the post-event vol crush.

How to trade it

Recognize the character: in a vanna-driven grind, dips are shallow and get bought, and fading strength fights a mechanical bid. Respect the grind rather than shorting it, and lean long toward the call wall while VIX is falling. When VIX stops falling — or starts rising — the vanna tailwind fades, and so should your conviction in the grind. It's context, one more layer on the dealer read.