Uncontrolled power off is a situation when the power supply is removed unexpectedly, meaning the power supply rails' discharge time depends on the input capacitor and load current. The corresponding situation is controlled power off, which means the power off is controlled by software, achieved by connecting the processor's power management pin to the power sequencing controller's enable pin.
In an uncontrolled power-off situation, the discharging time is short and may cause the power-down sequence to be out of order. The most efficient method is to use a supervisor to monitor the input voltage, and generate the sudden flag indicating power off occurred.
In this design, the supervisor monitors the input voltage rail. When the input voltage rail ramps down, the output of supervisor -- POWERONRSTn goes low, which disables the main oscillator, reduces the load current, and increases the discharge time.