Vanna

Vanna measures two equivalent things at once: how delta changes when implied volatility moves, and how vega changes when the underlying price moves. It is one of the cross-Greeks that explains behaviour first-order measures miss.

The Intuition

Higher implied volatility widens the distribution of possible outcomes, which makes distant strikes more reachable. That pushes the deltas of out-of-the-money options up and the deltas of in-the-money options down. Vanna is the rate at which that reshuffling happens.

Why It Matters in Equity Markets

Because equity index volatility is strongly inversely correlated with price, vanna effects compound in selloffs: price falls, volatility spikes, put deltas expand, and dealers who are short those puts must sell more underlying to stay hedged. This mechanical flow is part of why equity declines are faster and more correlated than rallies.

Practical Relevance

Retail traders do not hedge vanna, but understanding it clarifies why a short put spread can deteriorate faster than the price move alone would suggest during a volatility spike, and why risk in short-premium books rises non-linearly in stress rather than proportionally.