Hbridge Pwm Control Panel

I tested Hbridge PWM control panel. I don't understand why , when you start ezb and give a command (for example, forward) the motors do not turn at all, and you need to move the PWM sliders in some way to activate them. And, if you use arrow keys, and then the pwm sliders, the Arrow Keys are disabled. I expected that sliders were independant of other functions, and could be used, and saved, if necessary to a chosen value, without affecting other functions. Am I untrained and unable to use correctly this control ?