The drop is the voltage. If your driver is set to 3A, the drop will be 7.5V, if to 4A, then 10V. Works well - you can see Volts and Ampers at one screen.
I use it to set the driver current. Indeed, I did not find any test load which was described as behaving like a series of 2 or more blue diodes. It looked like all test loads were designed to mimic the behaviour of just one laser diode, so common voltage drop was in range of 3-5V if I recall it right.
I know this set does not look super professional, but simple things also work if there is no high tec stuff at hand when needed...