The Balans chair offers an alternative seating position which some find much more natural and comfortable than a conventional chair. Some with back problems will find that this posture can make desk time much more tolerable. I am not really clear on where the name came from, but it appears the original chair was invented in Norway in the 70s. It has been much copied and it seems that any Scandinavian furniture store or "Back" store has some variation on the chair.
Many of these copies are dubious in their quality, so I would not advocate anybody using one from a questionable source. Like any good task chair I would try to source one from a good manufacturer. HAG has offered a Balans style chair in the past and several online vendors still show them, however it seems to be absent from the HAG web site.
Here is the gist on the seating position. The Balans chair puts you into an upright posture, with your spine fairly vertical as if in a standing position. Because there is no backrest this is the natural posture in this chair. The knee rests support a small part of your weight, but for the most part your weight comes straight down your spine to your seat and avoids any curved spine positions as you might have while slouching in a conventional chair.
Many current task chairs offer a forward tilt position which approximates the Balans posture without the knee pads. Its also possible to assume this posture in any task chair by favoring the front of the seat pad and letting your knees drop below the seat level, with your feet folded under you. In this position your back is away from the back pad, your pelvis rotated around to a standing like orientation, and your back should be fairly straight and vertical. Whether you wish to work like this all day is moot - it offers you another position to freshen you and make you work more effectively. The forward tilt helps tremendously so look for this feature.