A twist bolster is a correction pad that is placed between the seat and base of your Sensation saddle. The purpose of the bolster is to give the saddle more waist. The more waist the saddle has, the less your hips have to twist out, which often happens with broad horse breeds.
Can be used for all saddles with a velcro system: Ghost saddles, Sensation saddles, Barefoot saddles etc.
You can se, how to place your twist bolster here