To learn the correct technique for landing the helicopter safely without the use of engine power. Nowadays it’s not usually taught “to the ground” in the PPL(H) syllabus, rather to the power recovery. It is often taught in continuation training, once the pilot is qualified.
Engine failure is simulated by reducing the throttle to idle, having first fully lowered the lever, and rolling the throttle control into the “detente” which uncouples throttle correlation to the position of the lever. Rotor speed is then adjusted by checking up an inch or so with the lever.
Note that during the actual landing, the low rotor rpm horn will usually sound. This happens because the rotors slow down as a result the rotors’ turning energy being converted into lift when cushioning the landing.
For more information on learning to fly a helicopter with me, please visit TimGilbert.com