There are two different kinds of sternum straps on the vests/vestas.  One is a hook and loop sternum strap and the other is going to be a sliding rail sternum strap.  The hook and loop straps have a t-hook that fits into loops on the vest/vesta to keep your sternum strap from adjusting when you are running.  The sliding rail sternum strap slides along a rail on the vest/vesta to get a more accurate fit.