Concussion should be absorbed in the ankle, knee, and hip of the rider..... NOT the "behind". Only the rider's correct leg & body placement (in a balanced saddle properly proportioned for him/her as well as the horse) can do this. The weight is in the heels.
Many SADDLES place the riders legs too far forward (stirrup bar placement and lowest portion of the seat among other factors) for extended distance work, greatly hindering his/her ability to keep the leg where it must be to save the horses back.
The saddle that is right for one rider may not be satisfactory for another rider with different leg length and proportions (upper leg length ratio to lower leg length). This leg ratio (as well as the other factors of saddle build) will definitely affect WHERE the rider sits in the sadlle when his feet are in the stirrups, as well as his balance when rising (either posting or riding "half seat", which is FAR preferable). Some of the riders look like even the seat size of the saddle was not correct for them.