Often, the rack is also called the single-foot rack. When performed properly the horse supports its weight on one leg at a time with lots of animation in the front and bearing down in the rear as the horse overreaches its stride. Standards do not normally have that high stepping action you might see in some of the other gaited horse breeds, such as a Saddlebred, Morgan, Icelandic, or Paso Fino horse, but for a smooth ride and the ability to turn on some speed they can't beat a Standardbred (STB).
I use a walking horse bit for balance, a short skirted western saddle and bell-boots in front. Some believe that a gaited horse needs special horse-shoes, or weights, but this is not so. My STB (pacer) mare is 100% natural and barefoot. To move my horse from the running walk into the rack I shift my weight slightly into the back of the saddle, slightly lift my hands maintaining a loose rein and then ask for more speed. The off-track Standardbred race horse, whether trotter or pacer, has to understand the saddle gait before I ask for speed. If my mare gets too pacey, doing the stiff necked, side-to-side, two-beat, hard pace she was trained to do in the race cart, I simply slow her down a little and ask her to lower her head a little bit, which smoothens out the gait. Not all Standardbreds will single-foot/rack, but most will. It is very important to allow the horse to build the necessary muscles, strength and coordination at the extended walk before asking for speed. A single-footing STB can easily reach 15-25 MPH, some go as fast as 30 MPH.
Below is an excellent reference website that nicely explains the foot-fall sequence, muscles the horse uses and the position of the rider. The author explains: "When moving the horse into the rack it is important that you ask ask for him to be collected first in the flat walk and then ask for the rack. You do not want to ask for the flexing of the neck after you go to the rack as it tends to pull the horse into a stiff straight neck. If you ask for the raised neck in a collected walk, the horse will be more likely to give you a nice rounded, flexed neck. The horse's center of gravity should be to the rear. The Racking horse is often quite fast, pushing a horse too fast will result in loss of gait. It is important that you do not push the horse past a speed where he can stay rhythmic and consistent in his gait." Here is the link: How to Ride Gaited Horses - The Rack