The FORCE JOYSTICK is just a better way to control zoom. It uses the exact same force sensor as on the Preston Microforce, Arri etc. which is THE industry standard way of zooming for movie work. The (A) side knob controls "zoom speed".
Without the force-joystick option... You can assign either the Slider or one of the A/B side knobs to control Zoom.
When operating zoom WITHOUT the optional force joystick, you can set the zoom motor to "Slow" mode so that its response is very slow and silky smooth, so operating in this way is better than it might sound!