Fingers are broken down into cylinders (phalanges) and spheres (joints) to make drawing movement easier.

If you're looking to dive into the techniques, you can find digital versions and deep dives on platforms like Scribd or Mangaestudio . La Mano en Movimiento - Mangaestudio

The book is famous for its "advanced" focus on hands coming toward or moving away from the viewer, using overlapping forms to create depth. Why it Matters

When fingers open or close, they don’t move in straight lines; they follow natural curves. For example, when you close your hand into a fist, the thumb always rests over the fingers, never inside.