https://www.youtube.com/watch?v=M8ExzaUq5GI

Here is a YouTube of a Reeves Drive in action. The two pulleys on the motor and spindle change in diameter. Thus changing the speed of the spindle. Looks like the guy moves a lever to change the pulley size. And get variable speed. Reeves Drive are a "cheap" way to get variable speed on a lathe or other machines. Cheaper than electronic variable speed. Electronic requires a VFD (variable frequency drive) and a 3 phase motor. Think the VFD takes single phase electricity on the input side and changes it to 3 phase to power the motor. The VFD must be able to change the amount of electricity that goes through and thus change how fast the 3 phase motor runs. I'm no expert on electricity and how 3 phase motors operate. Have no knowledge of Reeves Drive other than what it is and how it operates basically.