dual function lathe spindle

Gents

Tried searching this forum & the web, haven't found the answer to a nagging question. Hope someone here can help.

Depending on what machining operation you want to be doing, a lathe spindle could be either rotating at various speeds (vfd w/ an induction motor, pwm with a dc motor, or perhaps servo drive), or fixedly maintaining particular radial positions, similar to a 4th axis (which generally use worm gear reduction driven by a stepper).

An Emco CNC lathe with live tooling that was in a school shop I had access to a few years ago seemed to mechanically switch between these two drive systems to alternate functions, perhaps with electromagnetic clutches. However, lately there seem to be videos on the web showing both lathe and 4th axis spindles making this switch between functions seamlessly. The only sort of drive that I can possible imagine doing both would be a servo drive, but holding a static position would be problematic, as the servo motor, unlike a stepper, would be constantly hunting, wouldn't it?

What am I missing here?

TIA

Steve