These armor pieces feel like medium weight rubber. Inside each piece, there is a clear plastic shape imbedded which is designed to cushion the wearer from impacts. Go here for general information on armor of all types. Recommended for beginners, track school use, etc. The backing is thick suede cowhide and they are well made.
Plastic part has a medium thickness.
These are normally supplied on upper end Teknic suits, so they have better quality and life than standard plastic pucks. Read this guy's comments . You can have an alterations person attach the Velcro to your pants or suit for track use. It has adjustable straps for fitting around your waist. It is extremely similar in construction to the other high-end back protectors I have seen from other top manufacturers.