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After doing some research into what exactly second staging is ( sweatdrop ) and some practice in the test area, I've become fairly consistent with it.

For those who don't know, Second Stage is a type of quick boost that is much more powerful than standard quick boost. It is achieved by first making sure your QB is mapped to one of the triggers. It's actually fairly easy to learn. Simply pull back on the trigger smoothly. I can't stress smoothly enough. Not slowly, however, just at a comfortable speed. Make sure you pull the trigger all the way back, or you'll just trigger a standard QB. Practice it until it becomes second-nature.

If you've done it right, you'll be rewarded with a louder boost sound, a much bigger jet from your boosters (The 2nd Stage QB from my high-speed AC's side boosters easily takes up 3/4 of the screen), and a longer braking distance. To put in in perspective: With my high-speed AC, a normal quick boost to the right wouldn't get me anywhere near the edge of the platform you start the test out on. A second-stage QB blew me clean off of the platform.

I referred to a youtube video to ultimately learn how to do this:
http://www.youtube.com/watch?v=Jc76ADpXFug
KittyonEdge's Booster Tutorial(Also has a few other useful booster techniques)

It's hard to master, but, trust me, it'll be very rewarding when you achieve it.

I swear, I almost came the first time I did it

I've an interesting find to share.

Through more extensive testing, I've found that Quick-Boost has, in fact three stages. It's a matter of timing (though most of the time it simply comes down to luck for me) but it's attained the same way the second stage is.

I'd found that the jet from the quick-boost extended to one of three distances, depending on timing. Normal quick-boost distance, roughly 1/2 the screen, and roughly 3/4 of the screen, depending on timing.

I also found this:
http://www.youtube.com/watch?v=MDt7WsSgIvg&feature=related

To add to this. I was fiddling around on some random designs when I thought to try using the QB during a sword slash. Well it took a bit to get the timing down right but after achieving it I realized that you easily achieve 2nd stage via this method. So after experimenting on Soul Cage, my twin blade AC, I found that this method greatly increases the accuracy of my swings and allows me to easily cross the gap between me and my opponent. I highly recommend learning or at least playing around with this for anyone who uses a blade.