It's quite simple. The tailplane's primary role is to provide a downthrust in cruise flight to keep the cabin level. The upthrust from the airflow from below in autorotation is unwanted, hence the spoiler.
You can detect its effect if you enter auto gently with a fixed cyclic position. Initially the nose drops as the tailplane is unstalled in the negative sense. Then the spoiler disrupts the airflow over the top of the tailplane and the nose rises again.