Here's an aerial view. On the right ledge there's a quarter pipe ramp. You can actually do a complete flip in the Counter-Terrorist cars by driving off of it at full speed. Don't do it with your teammates in the car, though. There's a 50% chance that you'll land upside down and everyone will be crushed. There's a further 10% chance you will fall out and splatter on the ground. Plus a 5% chance that you will mysteriously die in mid air. Plus you if you hit one of the buildings you and your vehicle may get stuck high above the ground. Come to think of it, you might not want to do this at all if you like living.
One cool tactic I discovered is to drive full speed at the ramp and jump out of your car just before you hit. I've actually managed to have the empty car hit the terrorist truck and kill everyone inside. You don't get any credit for the kills, but it is hilarious.