One of the main ideas behind an institutional anchor is that it will be able to accomodate both a top-managed and a bottom-managed system from a single anchor. In other words, an instructor could facilitate belayed rappels and then transition easily to a toprope.
The following is an outline to the Backside System. This outline does not draw a complete picture of how it works, but will give you an idea. The components that I'm skipping in today's blog are those that help one facilitate a transition from top to bottom-managed or vice-versa. The best resource for a complete description of this system is in Rock Climbing: The AMGA Single Pitch Manual by Bob Gaines and Jason D. Martin (yeah, that's me!).
At its core, the Backside System is a classic "Joshua Tree" V system built with a static rope. Each leg of the rope goes to an anchor. These anchors can be different anchors or the same anchor depending on the anchor's quality. The system has two separate master points. There is a top-managed master point and a bottom-managed master point.
Drape the rest of the rope down over the edge and then pull it back up. Tie a second BHK in the end of the rope. This second point will be used for a toprope set-up.
The BHK at the top of the system will be used to facilitate top-manged climbing and rappelling. The BHK at the bottom will be used to facilitate toproped climbing.
The preceding is a very simple sketch of a complex system. However, if you play with this a bit, you may find it to be a very useful system for facilitating large group climbing days...
--Jason D. Martin