Colleges revoking acceptances?

Everyone says "you can have your college acceptance revoked if your GPA drops significantly," and I know this is true, but I am wondering which GPA everyone is referring to. I know any and all GPA's are considered, however is it the cumulative GPA, from all 4 years, that they look at the drop for, or only senior year compared to the others? I know they look at ALL, but can anyone tell me specifics please?