Extending the trot helps your horse with:
•Transitions
•Building muscles in the back end of the horse
•Increasing strength and suppleness in his back

The result is giving your horse the balance and ability to carry himself correctly through transitions.

Before you try to extend the trot, make sure your horse is responding to your leg and rein aids. Your horse must learn that leg means go! If your horse is listening to your added leg pressure immediately lighten your leg pressure to reward him. There are some horses that resist leg pressure by slowing down the pace or becoming stiff. To fix this, practice riding walk-trot transitions.

When asking your horse to move forward make sure they you are concentrating on your balance by stretching your legs and upper back making the center of gravity in the middle of the saddle. Follow his motion with your seat and soften your fists and wrists.

A good place to practice the extended trot in the ring is the long side and to shorten your stride on the short side.