Let’s say you are riding a horse in rising trot and the horse’s body is in right bend. You must post or rise out of the saddle when the left foreleg is moving forward and sit when the left foreleg is stepping on the ground.  This is posting with the horse’s correct diagonal movement to keep in balance.  Now let’s say your horse is still in right bend, but you are travelling left around the arena or ring.  In other words, your horse is in counter bend.  Most instructors I know will tell you to sit a bounce and post on the left diagonal which matches the direction of the arena.  But this will not make your horse a happy camper.  It will create discomfort and imbalance in the horse.  Please ride with awareness of bend.  Ride the horse, not the arena.