Zonal BOA® Performance. RIDE fit. The Synth is a mid-flexing all mountain boot designed for comfort and performance with a women’s specific fit. The Zonal BOA® Fit System allows you to tighten the upper and lower sections of the boot independently. Let’s break this down- When you pull the liner lace, it’s actually going to pull the top of the boot tight while securing the tongue of the boot over your foot and pushing your heel into the heel pocket. Stop reading and start buying. We promise, you’ll love the Synth.