Put simply willow just doesn't have the mechanical properties of a good bow wood. Yew is the best for a self bow (i.e. a single stave of timber) and spanish yew is even better. The sapwood is strong in tension and the heartwood is strong in compression creating an excellent bow. The sapwood is therefore the back of the bow and the sapwood the belly or the side that faces the shooter.
Oh , and whatever you do don't say "fire the bow" , irritates the hell out of archers, it's either shoot or loose.
