问个期权负gamma的问题

在猫大（https://bbs.wenxuecity.com/finance/6508941.html  ）和楚怀沙 （https://bbs.wenxuecity.com/finance/6508984.html  ） 贴的文章里，提到

“This dynamic will change after expiration, with traders transitioning from a slightly negative gamma state to a significantly positive gamma state. Historically, when the market shifts to negative gamma, the likelihood of a short-term rise in the U.S. stock market increases.”

请问这是什么原理，为什么过期后gamma会由负转正？