In the extended trot, the horse covers as much ground as possible. Maintaining the same cadence, with impulsion from the hindquarters, he lengthens his steps to the utmost. To get the most ground coverage and length from the frame, the rider allows the horse to remain on the bit without leaning on it.

In a proper extended trot, the front hooves touch the ground in the direction they are pointing; and the forward movement of the fore and hind legs are parallel while in extension. With well-balanced movement, the transition from the extended trot to collected trot is smooth with the horse taking more weight on the hindquarters.
 

Shown here is Anky van Grunsven on the late Olympic Cocktail. A KWPN approved Keur stallion by Purioso out of a Le Val Blanc xx daughter. Image taken by Dirk Caremans at the CDI *** Schoten, Belgium. 

See a video clip of the extended trot, featuring Gonnelien Gordijn-Rothenberger on Olympic Bo, a KWPN mare who was formerly ridden by Anky's life partner Sjef Janssen. 

Above definiton credit to: AHSA Rule Book 1999 Dressage Division Art 1904 Sec. 4.4 

Picture copyrighted by Dirk Caremans