The Flat Walk is in essence a sped up, more animated, normal walk. The horse will cover more distance by increasing the length of his stride, reaching further with each leg. 
To begin let's look at the horse and rider configuration. 

 

The horse's back will be carried in the Level Back position

The Horse

• The horse will be in a slightly rounder body frame in the Flat Walk than the Normal Walk. As the horse moves into the Flat Walk his shoulders will lift while the neck remains supple, his head should be just in front of the vertical. 
• The undulating motion of the back, (experienced in the normal walk) diminishes with increased speed and the back becomes more stable, croup will remain steady with no up and down bob.
• Overstride should increase from the normal walk, the degree will depend on speed and conformation of the horse.
• The head nod will be noticeably  more rapid than in the normal walk.

• The rider's position should be in the base position, until you need to make adjustments that will influence the horse's gait.

To ask a horse to change from a regular walk to a flat walk you want the horse to increase speed by taking as long a stride as possible, with an overstride with the hind. The average horse will do approximately 5-6 MPH in the Flat Walk. In the normal walk the horse's head and neck are stretched slightly forward and down, with the poll just slightly higher than the withers. To encourage the horse to bring the head slightly more towards the vertical, you will take LIGHT CONTACT on the reins. Simultaneously giving brief squeeze and release with the thighs and calves slightly behind the girth. to ask the horse to "move out". It is important that your legs make contact just at the point where the abdominals attach on the horse's torso, this will help a horse to round his back by causing him to tighten his abdominal muscles. Your pelvis will be in the base position.
 

Female Pelvis, Base Position.

Rider in Base Position with Good Equitation for Flat Walk.

The horse who has already been trained to perform a correct Flat Walk will increase his impulsion and overstride. His speed will increase because he is taking longer strides. Performed correctly the horse does not obtain the higher speed by rushing his gait, but rather by extending his reach in the gait of Flat Walk. You should feel and hear a 1-2-3-4 even cadence to the footfalls. The horse's head and neck will nod up and down in rhythm with the motion of his shoulders, back and legs. Your shoulders “move” with the shoulders of your horse, and your hips “move” with the swing of the horse's hind legs. 

Increasing speed, impulsion and reach at the Flat Walk
Sometimes it is necessary to encourage the horse to improve their Flat Walk. Make sure you are sitting in a balanced seat, with even pressure on your seat bones before you ask the horse to improve his gait. Crooked riders give unbalanced cues, therefor resulting in a horse not working in balance, working and pulling with his front end as much as pushing with its rear. 

Generate Impulsion
As the shoulder is moving forward squeeze and release the leg on that side firmly against the horse, slightly behind the girth. This will ask the horse to engage the corresponding hind leg. By alternating the leg application with the shoulder movement for 4 or 5 strides, the horse should begin to engage their hocks and stride out more purposefully with greater impulsion. It is important that you are squeezing, not bumping or kicking. After the horse has begun to increase the push with his hindquarters, you can ask for more stride length.
 
 

Increasing Stride Length You can increase the length of stride in the rear legs of your horse and encourage the horse to push harder with his hindquarters as he extends his gait by tilting your pelvis as you press your tailbone into the saddle, just as the horses hind leg starts forward, then resume the base position in a rhythm with each of the horses hind legs. You do this for 4 or 5 strides, then once again resume the base position.

Exercises
 
 

Virtually all horses can perform a respectable Flat Walk. Although the gait of Flat Walk is associated more with the TWH and SSH and Foxtrotter Horse breeds, it is an important gait for all gaited breeds to perform. The Flat Walk becomes particularly important to the breeds which perform the ventroflexed gaits as their signature gait. The rounder frame the horse carries themselves in to properly perform the Flatwalk can be part of a maintenance program to encourage future soundness in the ventroflexed horse. 

See Care and maintenance of the Ventroflexed (Hollow Backed) Horse for additional maintenance suggestions.

Achieving Response, Gait and Confidence through Relaxation

Rider Affect on the Horses Movement

MAKING CONTACT How to use a bit 

WORKING The WALK

Exercises at the Walk

Conditioning a Horse to Gait

Equitation for Gaited Horses

Exercises for Increased Flexibility in Gaited Horses

The Seat and getting the horse on the bit.