the key is that it has to hit the limit for the maximum amount of trim that should ever reasonably need to be added before it is disabled for the remainder of the flight. If it hit the limit it is an indication to the control system that something isn't operating correctly and it should stop automatically operating the outputs (trim) based on the inputs (AOA) as it isn't having the anticipated effect and it should punt the problem to the human pilots.

Presumably there is some annunciation for both the function activating and also being disabled.