Assuming it is similar to the loop-protect feature on other Procurve switches, it is independant from STP and much simpler.
Basically, the switch sends out loop-protect pdu packets on any port with loop-protect enabled, then if it receives the same packet on the same port, it disables the port.
Not nearly as robust as STP, but it's effective at stopping loops on unmanaged devices, where STP cannot operate.
Also, it is generally not vlan aware.
