When you start to pick up speed while mountain biking you have to be careful. The smallest miscalculation in a turn or jump can send you hurtling toward a tree. If you haven't noticed, trees aren't going to budge if you ram into them at full speed.
The video is funny; the guy was alright and walked away from the accident. However, many have died from hitting a tree while mountain biking. So stay safe, and don't push it beyond your limits!