Jason had never even considered what it would be like to fight against a mage who could control cause-and-effect, but now that he was seeing it right in front of him, he couldn't help but be impressed! Vana could literally change the concept of the universe so that his action and the reaction that results from it were under his control.
Whenever someone wishes to carry out an attack, they would usually need to first activate the rune they want to use, and then they would launch the attack with their body, and finally hit the opponent. But for someone like Vana, he could literally erase the second step of this equation and reverse it with the third step. Vana activates his rune, and the moment he thinks about launching an attack, the attack is carried out. So rather than it being that Vana launched an attack, therefore Jason was hit. Instead, the concept becomes that since Jason was hit, Vana must have launched an attack.
