Learning To Vanish
Once his mastery over mana threads reached an acceptable degree, Ben introduced Leo to the next phase of his training which was learning to use mana to interact with his surroundings, starting with the skill 'Vanish'.
This skill was a practical application of mana, requiring the practitioner to extend their control beyond themselves and manipulating the ambient mana in the environment.
The transition from manipulating mana within a contained object to influencing the invisible forces around him was akin to the transition from being a human from Earth, to becoming a being of the cosmos as once Leo understood this principle, he could grasp a new dimension that was impossible for humans from Earth.
Just like when manipulating the mana threads, initially, his attempts to extend his control outward were met with frustration.