Controlling the amount of running line outside the tip-top guide is a critical maneuver: too much of it and the cast collapses.
Why is it?
The difference in the mass / diameter ratio between the monofilament and the bead chain results in a bigger lose of speed on the part of the monofilament, so the chain overtakes the monofilament preventing loop formation.
Without a loop to give tension and "lead" the chain this latter goes forward wildly and not in a straight path, wasting its energy in many different directions and collapsing without reaching the target.
At least this is how I understand it.