It is designed to change the chainstay length by adding or subtracting 10mm. This offers you a choice about how the MTB fits or rides. Longer has more traction for climbing and is more stable when descending. Shorter is more lively and playful.  It also means taller riders can create a balanced front-rear proportion and tailor the fit of the bike. You can also flip the axle chip to long if you want to install 29 x 2.56 tires on your Tallboy. Unfortunately, the aluminum model of the Tallboy does not come with an axle flip-chip.