Well, most of that is handled by the software.. for example you have two faders and one button. The faders are set to control the volume of decks A and B when the button is not pressed and you can “map” them to control the volume of decks C and D when the button is pressed (just like shift on your keyboard) and in similar fashions (think caps lock for example).
As for faders jumping when you switch them, there is an option (in Traktor) called “soft takeover” which lets the fader control the level only after it had reached the point where the level was set the last time it was changed.
For example, your deck A volume is at 20% and you are controlling deck C with the same fader. The fader is at 100% when you press the button that changes the fader back to controlling deck A.. now the level won’t change until the fader reaches the point of 20% and aligns (so to say) with the level in the software and only after that will it resume controlling that value regularly.
(I hope I helped at least a bit, English isn’t my first language so I tend to get lost in some explanations 🙂 )
