The basic principle of how kneeling chairs provide ergonomic benefits stem from the way they encourage good posture.
Sitting in a chair in a kneeling position forces you to slide your hips forward thereby aligning your back, shoulders and neck.
By sitting on the chair in a kneeling position, your weight is distributed towards your shins for additional support, taking pressure off of your lower back and spine.
This also minimizes pressure points that normally exist while using traditional chairs.
The forward tilt from the chair seat itself reduces spinal compression, and as a result decreases the stress and tension in the lower back and leg muscles.
Kneeling chairs also force you to use your core abdominal muscles so you are not placing pressure on your skeletal frame.
Sitting in a kneeling chair will also help keep your spine in its natural "S" alignment.
Traditional chairs will actually place your spine in a compressed state, leading to slouching and poor posture.
By allowing the user to extend their legs and slant their thighs downward, your lower spine is relieved from excess pressure.
Kneeling office chairs force you to use your core muscles to take pressure off your knees and back, reducing compression.
An added benefit of using a kneeling office chair is the toning of your upper thigh and abdominal muscles.
There are some negative aspects of kneeling chairs.
They are best suited for short-term tasks that don't require a wide range of movement.
Your reach is reduced since it is more difficult to move the chair around, and because your balance is limited in the kneeling position.
If you are required to sit for extended periods of time, a more traditional chair would be better.