The programming terminal provides access to the FORCE instruction, which is a fairly standard instruction on most PLC's. By using the FORCE key, it is possible to test the input and output status of any device interfaced at the I/O rack.

For example, forcing a solenoid vavle ON will immediately tell you wether the solenoid is functional when the program is bypassed. If it is, the problem must be related to the software, and not the hardware. If the output fails to respond when forced, it is generally either the actual output module causing the problem, or the solenoid itself is malfunctioning.

Since FORCE commands must be executed when the system is fully powerd, it is necessary to use extreme caution when performing this type of test. However, the FORCE instruction is a very useful troubleshooting tool and is highly recommended for locating problems related to a PLC.

Indeed good maintenance culture is key,but we must not as professionals over look certain measures that will produce result that is harmless and preventing loss hours and delays on critical situations.I buy the idea of Brain by applying FORCE Measure on the Hard ware(solenoid valve).

I would not advice such this force instruction from terminal, because this can be used for training or small application.
For plants, professional way has to be implemented.
-First if there is good maintenance routine in place, no body will need to force the outputs nor inputs.
-The system should be provided with alarms so that the critical output should be provided with line monitor alarm.
-To carry out any forces of any I/O, it should be under certain procedure and it can be done from engineering station so that this action can be documented in the hostoorical event.
-Once the forcing or the overrried is completed, the document has to be signed off and handed over to operation or production so that they know that the plant back to normal operation mode.