When you have landed and stopped the engine, the rotor head continues spinning for quite some time until it stops by itself.
A rotor brake shortens the time you have to wait. Using light pressure with the flat of your hand on the domed face of the rotor brake you can stop the rotor in a few moments.
Pease note: stay in safe distance away from the turning circles of main rotor and tail rotor, and this applies to loose clothing as well!
CFRP rotor brake, designed for use with rotor discs up to 2000 mm Ø