The Elastic Strap Extender was built to provide extra flex in the spiritus systems back strap (purchase here) when the wearer needs a wider range of motion during high intensity activities or wants to layer up/down without having to adjust the length of the strap. 

To install the Elastic Strap Extender, simply remove one of the male Fastex buckles from the Back Strap and replace it with the Elastic Strap Extender.