@Daniel1984
Actually, in my experience, enema cushions are a very gentle and comfortable to really fill someone up. The pressure is comparable to a low hanging bag and the position allows the person to relax. If the solution has additives or is hot, the enema could be really painful, but that's true of any enema.
One thing does stand out about enema cushions. They are very good for mind games. A person can be told to sit on it and as soon as they tire and relax, the enema starts to flow. This can be a real attitude adjuster.
An enema bench is usually a smallish padded wooden bench, akin to a piano bench, fitted with an upwards pointing rectal nozzle. The user sits on it, bare ass, with the nozzle in and water is pumped upwards, usually by a simple piston pump. Pump details are unknown. Pressure from the pad presses against the user's anus, aiding water retention. I don't think the user expels the enema while sitting on the bench, but into a handy chamber pot or commode.
As too enema chairs, there may well be some confusion and fantasy. I suspect that the enema chairs were more for colonic irrigation than the relief of constipation or punishment. An enema given for constipation is usually designed to be retained for best effect. Retaining requires effort. Sitting upright, without a retention aid, makes this much harder. Also, there appears to be no mechanism for nozzle removal before pooing.
If OTOH the chair is actually for colonic irrigation, it all makes perfect sense. The design is perfect for the fill and flush at will of an open colonic system and the user poos around the nozzle.
Google Images is a useful research tool for pics of this stuff. The texts may not make sense, but look at the pictures. Our ancestors were not stupid. They made things to suit a purpose, hence enema gear was designed to be used. The design gives clues to the intended purpose.