These specials work by checking the floor and ceiling heights of the control sector. If floor height + ceiling height is different than it was in the previous tic, the scroller will move forward or backward accordingly, depending if the value is positive or negative.
EDIT: If either the floor or ceiling height of the control sector are changed, the scroller will move by (new floor + new ceiling) - (old floor + old ceiling), provided it's positive. If the value is negative, nothing will occur.
Since the operation performed is floor + ceiling, you need only to move one of the planes to achieve movement. If you move both of them simultaneously, the result should be double the speed.
Accelerative version adds the result to the existing value instead, so it will maintain its speed when the control sector is still, and speed up/slow down when it's moved.
In your case, if you want to make a button that activates a conveyor belt, try using an Accelerative line, and making the button's linedef executor change the height of either the floor or the ceiling of the control sector once.
I haven't tested this at all, I just surmised it from looking at the source.