Leaf fat specifically comes from the soft, visceral fat from around the pig's kidneys and loin.

It is very delicate, and has a super spreadable consistency at room temperature. This along with its clean, un-porky flavor, is why leaf lard is considered the highest grade of lard.

Rendering leaf lard is simple. You need one heavy pot, a spoon for stirring, clean leaf lard from a healthy pasture raised pig and some time. 

Leaf Fat or lard is the original shortening, way before Crisco was invented and for me, a much better source for the best pies in the world!