My lathe has six speeds, changed by two V belts and an idler pulley.  The idler pulley is driven by a toothed belt from the motor, so three speeds are driven by a V belt from the motor to the spindle and the other three lower speed from the idler pulley to the spindle.  Belt tension was by way of a idler wheel on an adjustable bracket, locked in place by an allen bolt.

Changing gears was a matter of releasing the tensioning idler, replacing the belt or moving the V belt then tightening the tensioning idler.  I quickly replaced that with a spring loaded lever which I pulled forward to relieve tension, changing or moving the belt, then releasing the lever.

Recently that gave way to another modification:  I remade the lever and pivot and installed a spring loaded hook on the handle, so now I simply pull on the lever, it locks into position via the hook, the belt can then be changed or moved and the hook lifted and the lever released.

Much, much quicker than the previous mod and much, much, much quicker than the original tensioning system.

While I was at it, I remade the carriage lock.  I installed a DRO a while back but that interfered with the previous carriage lock i made, so I made a new one with sits under the DRO.  I prevents the tailstock from being moved all the way up to the back of the carriage, but so too does the DRO, but, I can live with that.