The horses in ETG are not directly controlled by the player. Instead, the player controls the rider's actions, which the hors's AI then responds to. How the horse responds to the rider's instructions will depend on its personality and stats.
To the left, the player has the reins. These are used for steering to the left and right, for slowing down, backing up, for giving rein and for going into the two point (half seat) position.
To the right, the player has the leg aids. These are visualised as different contact points on the horse's body and represent different rider leg positions (right and left leg, in a standard or slightly forward position, or in a withdrawn position). Thus, using the front left button indicates a leg aid with only the left leg. Using the rear left button indicates a left leg aid with the leg drawn back, as if to ask the horse to move its haunches. Using the big middle button indicates using both legs in the standard position -- i.e., asking the horse to go forward.
Combining the reins and the leg aids, the rider can ask a range of different things from the horse.
Tapping the middle button once asks the horse to go forward. Single taps are useful when the player wants to fine tune the gait and not have a drastic change in speed. The player can double tap the button to ask it to go more forward. This is useful when the player wants a more aggressive acceleration.
The horse can be in one of 3 gaits: walk, trot or canter. We can think of each gait as a state. Each state has 3 sub-states: working, extended and collected. The sub-state says something about the momentum and the direction of the horse's energy.
Working is the default state. When the player taps the big button the horse will start walking, and it will be in a working walk (a High Spirited horse will however accelerate out of this state, and a Calm horse might come to a stop after some time). If the horse accelerates, the white dots will indicate that it is picking up speed. If it keeps accelerating it will soon be in an Extended Walk. In this state, the horse takes long steps and travels far with each stride. If it accelerates even more, it will transition into Working Trot.
Just like the walk, the trot can be extended into Extended Trot, and then transition into canter. The dots will fill up towards the right of the meter to indicate this.
But apart from extending, the horse can also be collected. When in a working trot, the player can collect the horse -- meaning it will maintain its gait (keep trotting) but also lose speed. It will use its energy more upwards rather than forwards. The player collects the horse through slightly pulling the reins to slow down, while tapping the middle button to keep asking the horse to maintain its energy and its gait.
In a trot, the player can collect the horse into a Collected Trot. If they keep collecting, the horse will transition into a Passage and ultimately a Piaffe, where the horse trots in place.
All horses can perform the extended and collected gaits, and passage and piaffe, but its stats in Walk, Trot, Canter and Strength will determine the quality of each gait and affect the dressage score.
Being in a collected or extended gait puts the horse's strength and stamina to a test and it cannot maintain this state for a very long time.