This new innovative patented design has an Active Cushion Sternum Plate that has two parts.  The outer is a reinforced plate that will not give under girth tension.  This is spaced away from the horse by the Active Cushion which holds the Sternum Plate away from the horse’s rib cage.  The Sternum Plate makes the girth straps bridge the pectoral muscles and it is this lack of pressure that is the key to allowing the horse to engage by the use of an unimpeded Rectus Abdominus and better leg retraction due to the Ascending Pectoral muscles not being squashed! 
The Standard Sternum Plate measured at the widest point, is 7.5 inches (19cm) wide.
The Slim Sternum Plate is for horses that are narrower between the front legs.
Measured at the widest point, the slim version is only 5 inches wide (12.5cm) so would be better suited for the slimmer horse.