Correct roundness of the horse is easy to see when observing the performance on a high dressage horse during a trot .
The horses back legs are well underneath him- rounding and lifting his back and arching his neck. thr horses front legs, This reduces weight on the front legs alowing the horse to respond more quickly and improves the horses balance.
As a beginning / intermediate rider the focus is on feeling and recognizing the roundness of the less advanced horse.
Observing horses trotting lose in a field, most horse will be some what hollow with neck and head forward and with there back legs to far out behind them.
The horse that has its hind legs more underneath themselfs , filling there back with and allowing them to arch there neck .
A horse ridden with incorrect reins ie from the head back will have a bend in the neck – caring the head vertically but with the back hollow or depressed and the hind legs behind him. This forces more weight on the forehand and results in an uncomfortable ride.
Riding a horse that is correctly rounded gives you a felling of lightness. As the horse rounds the back producing a slight side to side swaying at the trot, and not the harsher up down motion of a hallow horse at the trot. The reins are used to keep the head and neck strait so that so that leading can be done with other aids.
The horse head is not pulled back by the reins, but held gently and used to help keep the horse round.