NVRP measures the richness of option premium relative to the stock's actual movement: at-the-money IV minus realized volatility, divided by realized volatility. Positive readings mean options price more movement than the stock has delivered.
Readings of 0.10 and above mark genuinely rich premium; readings below zero mark options priced under recent movement, conditions that favor debit structures over credit ones. Unlike IV rank, NVRP compares against the current tape rather than a year of history, so it stays honest right after volatility spikes. The full treatment, including its event-risk caveat, is in the NVRP article.