The term comes from the type of controller used.
The original cars were Hi-V (High Voltage). The total voltage was channeled through the motormans controller. When he took on power it was the entire 600 Volts of DC. In the old days he could not move the controller handle to the multiple postion right away or he would blow out a good part of the trainline power system fuses. He had to take on power one notch at at time.
Lo-V means Low Voltage. I am not an engeineering expert but through whatever means used the 600 V is reduced to about 36 volts before it gets to the controller. This prevents any fuse blowouts when the controller is moved to the multiple position right away when taking on power.
I am sure this is an oversimplified explanation. Anyone care to go into greater detail?