Every time he attacks, it disappears, and Lang Ya disappears along with it.
But Qin Yi clearly discovered that his attack didn't really disappear; it was just repositioned. In plain words, it was Shifting Position.
Here, Lang Ya has the ability to change anything.
However, Lang Ya can't change the direction of his attacks but can change the four positions. For example, if Qin Yi strikes forward, Lang Ya can switch the front to the back. Therefore, Qin Yi's attack doesn't vanish but the forward attack becomes a backward one.
As for Lang Ya disappearing himself, it's because this is his Domain, and he possesses Instantaneous Movement abilities.
Lang Ya looked at Qin Yi arrogantly and said, "What, do you really think you can defeat me in my world?"
