How is an active output clamp implemented on a high-side MOSFET switch?
Simplest way is to put something like a zener diode structure from the drain to the gate. That way when the zener diode voltage is exceeded by a bit the MOSFET turns on and clamps the voltage.  Something like this: 







simulate this circuit &ndash; Schematic created using CircuitLab

If I read the datasheet correctly, the part you mention clamps at about -22 to -16V with a supply voltage 6~20V, so there's probably something a bit different going on- the zener is changing a logic level relative to ground (before a level shifter).