In some cases a freewheel unit and a selectable clutch are combined into a single unit. This is the system which I described in my previous post. This sytem is (was) used in Wessex Mk 5, Lynx and SeaKing, and probably may more helicopter types.
These "disconnect clutches" units are not commonly used elsewhere; I can't think of any modern designs using one. The Wessex one was "vulnerable" to mishandling (big bang and a new gearbox, please). Sikorsky has moved away from this design and prefers free turbines and a big rotor brake.
The Gazelle, which is equipped with a coupled turbine, has a ramp and roller freewheel and a centrifugal clutch. The engine can be run at idle (25,500 rpm) with the clutch naturally disengaged. As the throttle is advanced the shoes move outwards and engage the transmission, turning the rotors. The pilot had to monitor the speed at which engagement took place, it it was too reluctant it meant that new clutch shoes were needed.
The military Puma used to have a system whereby one free turbine could be disconnected so it's associated engine could be run without turning the rotors, but it wasn't selectable from in side the cockpit. It proved unreliable and if it slipped (it did), it caused the main rotor gearbox oil pump to stop so they were removed in the early 1970s, at least from the RAF ones.