Unfortunately the de-clutch mechanism proved to be unreliable in flight (!) and so the facility was removed.
The Puma de-clutch was a lever by the No.1 input drive on the gearbox. It had a locking pin that kept it closed and also to keep it open (drive disengaged). Two worthies, JA & MC, IIRC, failed to do this when they closed up and started the engine leading to some very expensive grinding noises. At that the facility was removed to avoid temptation.
Around the same time the RAF locked their cockpit doors so that the pilots had to crawl through the back to get at their seats; still a unique arrangement against the thousands of Pumas/Super Pumas around this world where they get through an convenient door.
The Super Pumas has an option controllable from the cockpit. The controls are similar to the Rotor Brake and Safety Lever mounted on the other side of the throttle box. A Safety Lever unlocks the disengagement lever which then slides the sleeve in the gearbox and disengages the rotor drive. The Safety Lever is then reapplied to lock it. The power turbine must be stationary during the engage/disengage process.
For those not fitted and on the Puma HC2 the outline** of the lever travels can be seen on the throttle panel.
** Always a good question to a trapper giving you a hard time.