In Backside Send the dog is sent behind the obstacle. The aim is that the handler can send the dog behind the obstacle from a couple of meters distance, or even further.
Advanced courses nowadays almost always include backside jumps, and many of the more complex handling techniques begin with a Backside Send. The more reliable Backside Send you have, the sooner you can start moving to your next position on a course, and the easier it will be for you to perform more advanced handling techniques.