Been trying to envision how a person would climb down a chimney feet first and IMO I think at first it would not be that difficult if one puts their back against the wall and puts their feet on the opposite wall to push their weight against their own back which keeps the person secured and keeps them from sliding down. I think the problem is that it takes leg strength and probably some arm strength to start to maneuver down. So a person could hold themselves for awhile but eventually the muscles in the legs would not be able to keep pressing against the wall and eventually a person would finally have to just slip on down until wedged. Ugggggg. This link below is not a real good example because the inside of the walls is far apart and they are doing it sideways with both hands and feet, and going up instead of down, but it gives you an idea how you can push against opposite sides of the walls to keep your body from going down.