A snaffle (direct rein from the bit) tends to over-flex them at the poll, and because a Walking Horses moves off somewhat faster than a novice gaited rider is accustomed to, the rider tightens up on the reins and prevents the horse from nodding correctly. Very shortly, the riders find that they now have a Racking Horse instead of a Walking Horse. Unfortunately, this is something that riders, new to gaited horses, seem to have to find out the hard way.
You should always ride your Walking Horse with a gentle hand, using only enough pressure on the reins to maintain a soft contact with the mouth. These horses have a naturally high head carriage, but require enough give on the reins to nod their heads as they perform their gaits. You should not try to increase the action of your horse's head by tugging or pumping on the reins. Nor should you try to set their heads by flexing them at the poll. This restricts the freedom of the neck, causing the horse to nod at the poll instead of from the withers. This will result in a shortening of their stride that produces a racking gait instead of a true, reaching running walk. If your reins are held gently, and the contact on the mouth is subtle, the nodding action of the horse's head will return a motion like a handshake, (hence the Handshaker name, that has been given to some of the best champions of the breed.)
Remember - if the horse is not nodding, it's not walking!!