Enstrom belt drive
Correctly adjusted, the blades should remain stationary with the engine running at the normal idle of 1400 to 1500. I own 1104 280C which runs as above. If your blades progressively run up to eventually match the engine speed at idle, (needles married) the belt drive system is NOT in correct adjustment.
I'm sure you understand the lower pulley rotates freely inside the belt loop with the manual clutch dis-engaged as does the jockey pulley. Some 'stickiness' in the belt can be improved with the infamous 'French Chalk!' Hope this helps, but Menominee is always the final source for type information.
Take care. DRK.