Our power meter HM8115-2 works like all other power meters, but the current will be measured in the ground path.
The voltage will be measured by the HM8115-2 between 1 & 4 and the current will be measured between 2 & 4. Based on this information the instrument will calculate the other values (e.g. active-, reactive- and apparent power consumption)
If you have fed the current between the two red sockets and the voltage between red and blue or black, then the instrument will only show the open-circuit voltage.