The pelvis is the crux of a good seat because of its facility to tilt back and forth, based on the flexing of the lumbo-sacral joint in your spine.
Pelvis Tipped Forward Or Back
Riding with the top of the pelvis tipped forward (and, unavoidably, the seatbones tipped back) is quite common. The reason most people do this is because they have never been taught the benefits of a properly upright posture with the weight going directly and lightly downwards on to the seatbones. It can occur through anxiousness to allow the horse to go forward or to simply go with him. When the pelvis is tipped forwards (hollowing the back) as a matter of course, rather than momentarily to give a halt or slow-down aid, backache soon ensues! It can also cause the rider to lean forward out of balance.
Conversely, riding with the top of the pelvis tipped backwards, flattening the small of the back, causes the rider to slump in the saddle, and once again the balance is badly affected. A pelvis with the wings or hips tipped too far back can simply be lack of knowledge of the correct posture, or a subconscious attempt to slow down or stop the horse.
With riders who know that a forward push of the seatbones is a request to go forward, it may be that the rider is doing this constantly to just get an apparently lazy horse to go. This pelvic position can be used as a speed-up or go-forward aid but only for a second or so, and initially accompanied by a leg aid, normally the inside leg.
The key to maintaining a pelvis in neutral, as I call it, is a knowledgeable eye on the ground. Ask a friend or teacher to take a good look at your position and posture. Once you have been put in the correct position and told that your posture is correct, get the feel of it into your consciousness and practise it until it becomes second nature.
It really does help to sit down lightly on the seatbones so that your weight feels as though it is falling directly, vertically down. Remember, up the body, down the legs.