I am sure someone will beable to reply with more details than me...but for now,,
It does what it says on the tin. I think it has 2 phases, an immediate phase, and then a slow delivery stage,
If say, you tested your BG before dinner, and it was 4.5...good, but, you don't want it to go down too much. Dinner is a relatively high far, medium carb meal...so, you are going to need some insulin, but you know, because of the fat in the meal, it is going to take a while for the carbs to get into your system. If you had all you dose immediately, you may hypo before the food had worked its way into your system. So, (and you can change the proportions and the rates with the meter...well worth playing with!) you could have say, half of your dose straight away, and teh rest of it dripping in over the next hour. It is really useful if you get post meal spikes as well.
Jopar is far better at explaining this, but above is my "simple" explanation. Liek all things with teh pump, it is worth trying out, and seeing how it works (or doesn't) for you.