About the simplest analogy I can think of is this:
A traditional "Speed Coil" TM is like driving a a stick shift with the gas pedal floored and controlling the speed using the brakes. The main drive coil is energized as soon as the motor is powered. In addition, there are multiple speed coils that are energized in different groups to give you the 1-5 speeds (At 5, no speed coils are engergized, at 1 they are all energized)
The digital control varies the amount of power sent to the main coil, and no speed coils are used.
If you noticed Bowhunter's conversion article in the past few days, what he has done is taken the speed coils out of the system and added a digital control in-line with the main drive coil.
The digital maximizer is helpful if you tend to troll or move for long periods of time at a slower speed. If you are the type of person who pulses the motor on & off to move quickly from one spot to another, the effects of the maximizer aren't as noticeable.
Good luck!