Here is a quote from an article on sights that Bob Robb wrote for Bowhunting.net.

Pendulum Sights
Designed expressly for the tree stand hunter, pendulum sights feature a single horizontal cross wire or pin that is permitted to freely pivot on a hinge so it rises as you take aim closer to the base of your tree and drops as you aim farther away.

Together with a fixed vertical stadia wire, the two give a precise aiming point out to 30 to 35 yards, the exact distance being directly proportional to arrow speed. Beyond that distance the system breaks down. Some pendulum sights attack this problem by adding a couple of fixed horizontal pins or stadia wires to give the shooter an aiming point at longer distances.

The better pendulum sights will allow you to adjust the length of the pivoting arm thereby fine-tuning the sight for your own individual arrow speed. However, sights without this feature give acceptable accuracy at the distances at which they are designed to be used. One disadvantage of some pendulum sights is that they tend to be a bit noisy and their moving parts can break or stick. There are several good pendulum sights on the market, including those by ABC Pivitol, Predator Products, Keller, Saunders, and Advanced Archery Products.