The common safety circuit consists of protection IC, Mosfet, insisters and capacitors,NTC/PTC.
When the cell voltage is between 2.5V to 4.3V, the first and third foot of IC output high voltage (equal to the supply voltage), the voltage of second foot is 0V. At this time, the first and third foot voltage of IC will be added to the fifth and fourth foot of MOSFET respectively. Two electronic switches in MOSFET are in conduction state(open state) because its G pole received the voltage from IC. This moment, the cell’s negative and P- end of protection board are directly connected, so the protection board outputs voltage. See below diagram