Remote voltage sensing . Four conductors in the cable. Two carry the power ( high amperage and voltage drop ) to the lamp. The other two, the sensing pair ) return the voltage at the lamp to the control circuit in the power supply, there is almost no currrent flow on these so no voltage drop. The power supply uses the sensed voltage to adjust its output so as to have the correct voltage at the lamps. Some remote sensing power supplies can operate with a single sense wire and a bit of electronic "mathmatics" in the controller.