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The schematic arround Q11 introduce a delay between Vlow goes high and UVP1/UVP2 low.
When Vlow appears, C11 is discharged, so Q11 is blocked. Voltage on Q11 collector is arround Vlow value. UVP1 and UVP2 are high (arround Vlow value). C11 charge through R42 (time constant), increasing voltage of Q11 base. When voltage on Q11 base is arround 0,7V, Q11 go into a staturation state and voltage on Q11 collector fall arround 0,5 V.
Diode D13 helps C11 to quickly discharge when Vlow desappears. C11 dont discharge through R42, R43 and R44, but through D13. So Q11 go to a blocking state before Vlow reach a very low value, allowing UVP1/UVP2 to go high (Vlow value).
If you have pSpice, you can simulate this section of the schematic. C11 initial condition must be 0V (discharged).
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