Ok, I've looked on the walkthroughs and FAQs and NONE of them say how to do the Slingshot. I did it once, on the way to MBC, or whatever it is. What i'm asking is, can you slingshot any time you want? Or can you only do it when you need to?
I've tried that. It won't work on the vents on top of the buildings. And when I look at it in the upgrades menu, it just says "Works in special areas". So, I can only use it when the main story lets me?