Unidirectional Carbon (UD) is used to add stiffness to the board without affecting the board's thickness. Only an average of 50g is added to the board's total weight.
A single strip of UD runs from the nose to the tail and helps strengthen the board vertically. At the same time that your board's pop will increase and last longer, the board's flex will decrease. This kind of strip can be used in everything from freestyle and dancing boards to downhill boards, and is an ideal, lightweight alternative to adding a wooden ply for stiffness for you heavy riders.
On top of flexing from nose to tail, skateboards can flex from one edge to another, kind of like that ruler we all used to have in highschool. That is what we call torsion. The cross is two strips of UD which are laid across the board from shoulder to shoulder to get rid of torsion which can be distracting at higher speeds. This option is ideal for riders who want to turn their board into something that can handle HEAVY hills with no play in the board for ultimate stability.