Now, for libertarian free will to be true, the will's act in causing your legs to carry you to the kitchen cannot have been caused by something other than the will. That is, your will, by an act of will, caused itself to move your legs by an act of will. If act of will moving the feet was caused by something from outside the will, the desires of the heart (like we Calvinists claim) or genes or social conditioning or what have you, by definition we'd have determinism.
Understand so far?