Ergonomics is a science that studies you, your job conditions and the tools you use to make a safer, less injury-prone work environment. This is done, for example, by examining your posture while sitting, and measuring how far you are from your computer keyboard and monitor.
According to ergonomics-information.com, the problem with traditional chairs is that they are inflexible and encourage you to slouch, which can cause back pain.
A kneeling chair does not have a back; instead of your feet sitting on the floor, your shins rest on a pad. The majority of your weight should still be in the seat, as in a traditional chair, but the seat will slope forward at 30 degrees.
A kneeling chair encourages your abdominal muscles to work to support you while you sit, distributes body weight evenly and aligns your spine in a balanced position, which places less stress and strain on your back, neck and shoulder muscles.
While in a kneeling chair, your spine is in its natural "S" alignment, which relieves back pain or pressure. Because you are moving pressure from only your bottom to your legs as well, the chair helps relieve soreness and improve circulation.