It is comprised of 4 sensors, one mounted on each of the Y-axis motors, one on X and one on Z. The sensors monitor the motors’ movements and compare that movement against what UCCNC (your project’s G-Code file) is telling the motor to move. In the event that one of the motors does not match the intended movement, then the system will immediately go into E-Stop.
As your machine is cutting a project, if it runs into an obstruction, or if there is any other situation that causes the machine to stall in one of the axes, it will stop the system.
To reset, you can simply rehome the machine and then continue running the job from where you left off. You can use the RUN FROM HERE button on UCCNC and the job will continue as if the error never happened. You can watch the following video to learn how to use this feature on UCCNC: