問個期權負gamma的問題

在貓大（https://bbs.wenxuecity.com/finance/6508941.html  ）和楚懷沙 （https://bbs.wenxuecity.com/finance/6508984.html  ） 貼的文章裏，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.”

請問這是什麽原理，為什麽過期後gamma會由負轉正？