"That is the difference between a Heaven grade skill and the ones you have practised so far. Starting from Heaven Grade, the more mastery and understanding you have over the skill, the more versatilely you can use it" Amon explained, sitting on his head.
This explains it. So the reason why Xing Xia was able to utilise the destructive energy of the [Gauntlet of Radiance] to increase the lethality of her sword was due to her understanding over the skill.
No wonder she was the senior sister of this academy. As someone who also practised numerous Heaven grade skills, he understood the complexity and profundity involved with each of these skills.
Thankfully, he had the system to manage all of this for him, so he only needed to practice it for a certain amount of time. However, for others, they could only forge ahead while blindly practising and hoping to gain an enlightenment to increase their mastery over their skill.
