If you enable it, the FailSafe will be triggered if the following situations occur during flight. According to the difference of aircraft height, there are two situations.
In the GPS or ATTI. Mode, if the requirement 1 or 2 is satisfied, and the output data of channels A/E/R/T have not changed for 20 seconds, then the aircraft will hover automatically. After that, if the output data of channels A/E/R/T still do not any changes and last for 10 seconds, the autopilot system will think that the data from receiver is abnormal, and then enter the FailSafe Mode.