Gamma

Gamma is highest for at-the-money options close to expiry, which is why a short ATM straddle held into the final session can swing violently on a move that would barely register a session earlier. High gamma cuts both ways: it's what makes a long option's payoff curve bend upward so favorably, and what makes a short option's exposure so much harder to sit through near expiry.

Example: an ATM option's delta might move from 0.45 to 0.65 on a single 100-point spot move in the last session before expiry. The same 100-point move a week earlier might have shifted that delta by only 0.05. The option itself hasn't changed, only how much gamma is in play that close to expiry.