The hind quarters of the horse on the left stick out of the plaque, catching the light and directing your eyes straight into the scene in a clever trick of perspective. Other horses, pulling a chariot containing a victorious Roman emperor and general, have been compressed to appear further away.
It takes great skill to produce high relief decoration on an essentially flat object like a plaque. The design for this plaque would have been pricked out on the surface of a flat sheet, then the sheet turned over and the basic pattern hammered out from the back. The detail was worked in from the front using a variety of hammers and punches, with the sheet resting on pitch to hold it steady.