there are two main variations of the underbar. The 360 underbar, and the regular "underbar". In any underbar variations, you perform on an obstacle anywhere from your head level (if you are really good) to your lower waist level (if you are really good)

.Also, you are going to be going under the obstacle.
When doing a 360 underbar:
As you approach your obstacle, lean forward or dive forward while turning 180 degrees and grabbing the the bar (your hands will be in a pull up type of grip). Then, try to keep your momentum going as you pull yourself up. Once your back is clear of the obstacle pull your knees to your chest and turn another 180 degrees (on the x,z plane) and land, then keep running.
In the regular "underbar" ;
Jump up almost parallel if not slightly higher than the object, with legs bent partially in towards the body. Then, synonymously, grab the intended ledge above the gap, with both hands, and thrust the legs through the gap while pulling your body through the gap on as flat a plane as possible. The arms should pull as if pulling the ledge right across the chest and past the head. After the space has been cleared all that remains is a basic landing, a good flick of the upper body propelled by the arms off the ledge can help to re-align the body for landing