The world expert on gliding in snakes is Jake Socha; he worked under Mike LaBarbera at University Chicago years ago, and now has his own biomechanics group at Virginia Tech.
Jake's snake work was featured on National Geographic, and the program is actually available in full version on YouTube now, albeit with some ad breaks: Snakes That Fly
As a quick synopsis, essentially what happens is that Chrysopelea individuals can glide by flattening themselves (spreading their ribs) and moving in a repeated S-curve that spins counter-vortices over both sides of their bodies, creating a low pressure zone over their backs that allows the snakes to glide. My understanding is that the snakes essentially drop to launch, building speed and then pulling out in a large J-hook, though they may add some spring to the initial launch by unloading from a coiled position. It should be noted that this is the exception that makes the rule: every other flyer, both gliders and powered flyers, that has been examined in the lab uses some kind of leap or run to initiate flight. Snakes, of course, are a weird case because they lack limbs with which to do these things.