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The real plus for this propulsion technology is being able to vary or modulate the plasma exhaust while maintaining maximum power. This technique works like the function of the transmission in a conventional automobile. That is, you have engine power either for speed when driving on a level highway or for torque over hilly terrain.

Two parameters are varied during a typical engine operation: thrust and the velocity of the particles being exhausted. This latter factor is called the specific impulse. As a VASIMR ship accelerates on its journey, the thrust decreases and the specific impulse increases. The opposite is true as the ship slows down at its destination.

This has me a little confused. Could you explain how this works and why it would be important?

I'll try to summarize:

Thrust is for power, and is used to great effect to get mass to move at all. Imagine that it is like the low gear in a car. You use it to get going, but your speed is limited.

Low Thrust with high specific impulse is advantageous when you've already begun moving. Imagine that this is a car's overdrive, where you can't accelerate very much, but you can maintain your current speed with ease. For rockets in space, this actually means maintaining acceleration at some level. It may be a small acceleration, but the fuel expended to sustain it is very low.

The high specific impulse is the top single property of the VASIMR, in that you sip fuel while accelerating to your destination for the entire trip. Conventional rocketry runs through fuel quickly, and then you are stuck with some final speed. VASIMR rockets, OTOH, add speed for the whole trip, making your top speed phenomenally higher compared to conventional systems.

Of course, the second beauty of the VASIMR system is that the system can "change gears", if you'll allow me to extend the analogy further. About halfway to its destination the rocket turns around and accelerates the other way, slowing its descent. As it get closer and closer to orbit insertion, the thrust gets stronger. Imagine down-shifting to a stop light, and you'll get the picture.

There is one more thing that's nice about being able to modulate between thrust and high specific-impulse, and that is the arena of abort scenarios. Conventional rockets pretty much leave the system high and dry in this regard, because the bulk of their fuel (if not all) is used simply to get going. VASIMR, it's much more survival-friendly. Even at a very high-specific impulse limit, a VASIMR can limp home if need be. On the other end of the spectrum, the high-thrust mode is available for dodging debris, and then resuming course.

Less confused, I hope?