Roll up to the obstacle at a slight angle with your feet in ollie position and your knees bent. Ollie up, and aim your trucks toward the edge of the obstacle. Try to land mostly on top of the obstacle with your weight centered over your heels and your wheels locked up against the edge. Grind until the end or until you're ready to come off. Shift your weight over the back truck, and press down on the tail, lifting your front truck off the obstacle. Use your momentum to get your back truck off. Land, and roll away.