You can think of the Semi-Closed Loop System as an “electronic insurance policy”. The system uses a special sensor that is installed on each of the 4 CNC motors. This sensor monitors the movement of each motor and compares that movement against what UCCNC is telling it to move.
As an example, if UCCNC wants to move X 35mm to the right and something is interfering with that movement and it only moves 31mm, then the Semi Closed Loop System will immediately stop the system.
The beauty of this system is that you can simply rehome the machine and then continue the job right where you left off. This is especially handy when you are using expensive materials that you do not want to waste.