A pendulum sight is primarily designed for treestand hunters, as it helps to compensate for elevation on downhill shots. The pendulum, or treestand sight, usually has a single pin which is mounted on a hinged pendulum swing. As the bow is tipped forward (shooting downhill), the sight pin swings out and up, automatically compensating for your downward shot angle. The steeper your shot angle, the more the sight compensates for you.
Most pendulum sights are advertised to be accurate from 0-30 yards (within a typical bowhunter's range). And it's worth noting that the pendulum sight is not as effective for longer range shots and/or when shooting from the ground in some/most cases. It all depends on arrow speed as to how far your effective range can be. Faster bow, longer range. Slower bow just the opposite. I can lock mine down so I have a 20 yard range on the ground.
But if I were to use a fix pin, I would get an HHA.
I tried to find video of how it is used, but failed!! might be a good project for someone.