A long time ago, a Mr McMahon published his design for what he called a "Psuedo Clutch" on his Warco BH600. in MEW. m Which sounds like the device alluded to, above.
Am ashamed to say that on a dfew occasions, I slipped the belts!, (Did not immediately apply full tension ) on my ML7, as a form of clutch.
With the VFD on the present machine, the belt is just tightened fully, and the VFD slow ramp up takes care of things; or the "jog" facility is used.
Howard