Relaxation/looseness - the horse should have no tension in its body at all. It should be swinging through its back, snorting (a snorting horse is a relaxed horse) and a closed but not motionless mouth. A relaxed horse will stretch its head and neck forwards and down in walk, trot and canter.
Rhythm - every stride in every gait should be the same as the others. One trot stride should not be longer or quicker than any other stride in the trot. This rhythm should be maintained on bends and on straight lines, and the horse should not need to be continuously reminded to maintain rhythm. It should continue at the speed the rider puts it at until the rider asks for a change. Tempo is the speed of the rhythm; how fast the horse is moving is feet and leg. Cadence is when the horse is showing much rhythm and appears to have more 'lift'.
Contact - the rider should be able to feel, but not be pulling on, the horse's mouth. The horse should be willingly stretching into the contact, but should not be making the rider 'carry' its head. Correct contact can be achieved when the horse is coming forwards with its hind legs, which is shown when the horse has lowered its head and is keeping its own rhythm.
Impulsion - the willingness of the horse to go forwards. The horse should be working from its hind legs, not its shoulders. It should travel forward with energy and should swing its feet. It should happily accept the rein aids, but it should remain in front of the leg, and should not need to kicked on to maintain impulsion. Note, impulsion is not speed, nor is it forcing the horse out of its natural gait.
Straightness - the weight of the horse should be evenly distributed over the two sides of the horse. Straightness does not mean the horse is like a ruler, but that the dorsal line of the horse follows the track it is going on. It should be able to turn with equal ease on both sides of the body. Because horses (like humans) favor one side of their body work needs to be done to allow them to push from both hind legs evenly up through the horse. Straightness can be tested through the shoulder-in exercise.
Collection - this happens naturally through the correct training of the above skills. The horse will be carrying more weight on its hindquarters and so be pushing with its hind rather then pulling with its shoulders. A horse that pulls will put its head up, a horse that pushes will put its head down. The result will be a horse that moves with a constant rhythm, with balance and with self-carriage in walk, trot and canter. This horse will be said to be on the bit.
References:
- The BHS Manual of Equitation (2007 edition)
- Various riding instructors over the last few years