Here's how to do it: Approach the obstacle, parallel with it, (either frontside or backside), and ollie up to it as if your were about to 180 ollie. As you reach 90 degrees, your tail and back truck should be over the ledge. Push down on your back foot so your tail will smack down on the obstacle, put all of your weight on your back foot, and you should start to slide, remember to keep your balance with both of your feet, come off either forward of fakie, and ride away smoothly.
You might be a little confused about this trick so it's important to watch people performing it for a better understanding.