I personally haven't done it myself but the basic concept looks like this: when you submit AMCAS as you would submit it in the late May-early June timeframe, you pick and pay for 1 school that you think you likely would not get into (classic examples would include Harvard for most students, UWashington Seattle if you're out of their 5 state region, etc.) or simply would not ever want to go to. This gets your primary app in the queue for verification. After you get an MCAT score back then you can go in and add your school list, as once your app is verified your app is sent instantly to any new schools you select.
Basically the point of this is to make sure you start getting secondaries without having to wait a month+ between MCAT score and primary app verification