Why did a Chubb lock remain locked if a tumbler was raised too high?

The Chubb lock remained locked when a tumbler was raised too high because it incorporated a security feature known as a "detector".12

Patented by Jeremiah Chubb in 1818, this mechanism utilized a retaining spring that would catch and hold any tumbler that was lifted beyond its correct height during an unauthorized attempt to pick the lock. By trapping the over-lifted tumbler, the detector made the bolt immovable, effectively preventing the lock from being withdrawn. Additionally, this feature served as a visual indicator that the lock had been tampered with.21