Negative drain-source voltage
It acts as what it is: the body diode in parallel with the MOSFET source-drain resistance.
Current is shared between the two until Vgs is high enough that the voltage across the source-drain resistance falls below the forward bias voltage required to allow the diode to conduct. At this point, the source-drain shorts out the body diode and all current flows through the source-drain, rather than the body diode, and has a resistive-only voltage drop associated with it.