Byzantine Weapons and Warfare

[Urselius]

From the context, the word κάλπα means a type of canter. My guess would be that it means a "hand canter", this is a slow canter allowing horsemen to maintain a close formation. A faster canter would tend to break up a tight formation. Also with the shield being held so far forward it would tend to strike the horse on the neck, which would startle it with no good result for an attack.