How over-discharge protection function works on this circuit?
That board looks like a DW01 IC based board. Let's look at the DW01 datasheet.

It has these diagrams:



Note that the BATT+ and BATT- connections in this schematic correspond to the
terminals marked P+ and P- on your PCB!

Also note that the IC detects Overdischarge by monitoring the voltage of the battery using the botton left comparator in the Functional Block diagram.

If the battery's voltage gets too low, the DW01 will switch off the two MOSFETs (M1 and M2) to open the connection to BATT- (= P- on your board) to stop the discharging.