For a given expiry and tenor, implied volatility varies across strikes.

Two core metrics stand out:
=> Skew (The Slope): Reflects the market's directional asymmetry.
Rich payers: Indicate that the market is aggressively hedging against surging rates driven by inflation fears or upside shocks.
Rich receivers: Indicate that the market is paying up to hedge against a growth slump, recession, or deflationary spiral.

=> Vol-of-Vol (The Curvature): Measured via a volatility butterfly: σ(payer)+σ(receiver)−2σ(ATM).
A rising butterfly means the wings (out-of-the-money options) are getting expensive relative to at-the-money options. This signals that institutional flow is aggressively buying tail-risk protection, leaving market-makers short those wings and exposed to volatility shocks.