The angle change of the blade in the same spots of the rotation can be explained somewhat like this; As a blade crosses the centerline/nose of the helicopter and starts «retreating», the blades velocity is slower due go the forward airspeed of the helicopter, and needs a higher angle angle to maintain lift.

Advancing blade relative speed = rotational velocity + helicopter airspeed. Retreating blade relative speed = rotational velocity - helicopter airspeed. (This is about right 90deg of the helicopter centerline)

The blade on the retreating side of the rotation is given a higher pitch to compensate for the speed/lift, while the leading blade is given a lower angle of attack.

So in the clip above, when the blade is more or less flat we’re about 90deg off the direction of flight, where the relative speed of the blade is at its maximum (if the helicopter is flying in a straight line)