Refer to figures. 
To enter in a vertical descent with power on, the collective control must be lowered, thus, reducing the main rotor blades' pitch angle, than it was in straight and level flight, with the Weight being now in excess of Total Rotor Thrust.

Now, the Induced Air Flow (IAF) is opposed by a rate of descent airflow and is thus decreased. With the decrease in induced flow, the angle of attack increases and becomes greater than the pitch angle.

The total reaction vector is tilted forward, towards the axis of rotation, because the direction of the relative airflow has been changed and the pitch angle has been decreased. The Total Rotor Thrust has been reduced slightly, as the result of the Total Reaction tilted forward and the decrease in pitch angle.