What is the purpose of this NPN transistor in a battery protection circuit?

Question

I found a circuit using a HY2120 2-cell battery protection IC. The application notes differ from how it has been used in the circuit below and I don't understand why the author has not just directly connected the VSS pin to ground.

What purpose does the sub-circuit in red serve? Has it got something to do with the fact that B- and P- are both connected to GND so the protection wouldn't work if the IC was still connected to P- when the protection triggered?

Circuit in question:

HY2120 application diagram:

Huisman do you mean something like the following flow if P is reversed?

Answer 1

I tried to redraw the schematic so it matches the HY2120 application diagram.

^{simulate this circuit – Schematic created using CircuitLab}

Instead of using the R_DS(on) of (Q3||Q4)+(Q8||Q9), the author decided to use R14||R15 as current sense resistors.

I don't understand why the author has not just directly connected the VSS pin to ground.

There is no ground reference in the author's schematic. But there is indeed a reason the author separated the B- and the P- connections.

B+ and P+ are connected. If B- and P- were connected as well and a reverse voltage was applied to P+ and P-, then this reverse voltage is also directly seen by the batteries.
The author added 2 reverse polarity protections:

1. D1, a 1SMA5924BT3G, a 9.1V Zener diode of 1.5W which will be forward biased and will (temporarily) clamp the voltage to some forward voltage (about 1.5-2V??) and will hopefully trigger a circuit breaker before the zener blows (because the forward voltage is already 1.2 V at 200 mA, it is likely it will blow quite easily).
2. The red encircled subcircuit. I think the author makes use of the internal clamping diode of the HY2120. (I assume there is a clamping diode based on the Absolute Maximum Ratings: Input voltage between VDD and VSS pin: VSS-0.3 to VSS+10 V)
   Because P- is positive wrt BP+, D2 and the clamping diode are forward biased and Q7 will turn on. Q7 makes sure mosfets Q8 and Q9 are turned off. They have to be turned off, because the body diodes of Q3 and Q4 are forward biased.

Although it probably works, I think using an unspecified clamping diode for this protection circuit is bad practice.