Charm is the time decay of delta. As an option approaches expiration without moving in or out of the money, its delta drifts toward 0 or ±1 deterministically. That drift forces dealers to adjust hedges throughout the trading day — particularly in the afternoon when charm is largest for short-dated options.
Charm is the engine behind the late-afternoon pin: as 0DTE options decay, dealer deltas migrate toward an equilibrium that — combined with positive aggregate gamma — produces the slow grind into the close that traders see most Fridays and OPEX days.
Charm = -∂Δ/∂tHow traders use it
Combine charm with the gamma profile to anticipate end-of-day moves. When aggregate gamma is large and positive and charm is concentrated near the at-the-money strike, the close is statistically biased toward pinning the largest open-interest level.