Throughness

In equestrianism, throughness is an absence of resistance in the horse to the rider's commands.

A 'through' horse is perfectly submissive, allowing the rider's aids to go freely through the animal, with the reins influencing the forehand, and the riders' seat and legs influencing the hindquarters.

When completely through, the horse is soft and elastic, with a connection from back to front, balanced and relaxed.

[1] Throughness is often compared to a circuit of energy between horse and rider: the rider's leg aids encourage energetic movement in the hindquarters, which push the back upward, which in turn allows for connection with the front end and the bit, and the connection felt in the bit transmits a feeling of energetic movement back to the rider's hands.

[1] Of course, this is a question of "feel", meaning a very soft reaction in the rider's hands.