Sounds like the belts are out of adjustment and when off, they don't loosen enough. Look for missing springs or too tight cable from the PTO (which your lever to engage the mower is connected to).
Also obstruction that is blocking normal movement or anything sticking. When you turn the big lever to the engage position, the belts should tighten on the pullies and the mower blades turn. When you turn the lever to off, everything should relax and the motor PTO pully should spin without engaging the fan belt.
Hope this helps.
Bruce
Bruce
May 2008