The 3 piece design creates a more even 'linear' flex that is determined from the stiffness of the tongue instead of how tight you buckle your boot. Among other reasons. This flex helps combat shin bang and the design is simple so they are much lighter than most boots in the freeride & freestyle market. Thanks!
This video can explain a bit more. It's mildly old but still relevant! Kinda like our original FTO shell ha!