Hi Kamilb,
A throttle is basically a variable resistor (1k-10k restistance) which uses three wires. This normally adjusts a voltage into a controller (0-10v, 0-5v). This variation of voltage ramps the signal via electronics to the output of the voltage or current or frequency. In the case of the 24v Uni8 this modulates via PMW (pulse width modulation). This basically gives 24v in pulses (on / off signals), were the shorter the pulse faster the motor (think i got that right - long time since went to college). You cannot do it with the standard control box as it basically energises a relay to give voltage / current to the motor.
Hope this helps.
Lance