The Heart-controlling Technique is indeed the next stage of the Soul Searching Technique, evolving from observing someone's mind to controlling it.
Of course, this Heart-controlling Technique requires considerably more power than the Soul Searching Technique. The key point is that it can only be done when there is a significant disparity in strength between two individuals.
With Su Yang's current strength, although he is also a Sovereign, the fact is that the Sovereign is part of the Integration Realm. He currently cannot use the Soul Searching Technique on other experts in the Integration Realm, only on those below it.
And the Heart-controlling Technique, being even more difficult than the Soul Searching Technique, is limited to those below the Qi Refining Realm.
This short man can only be considered a minor figure skilled in martial arts, not even reaching the Foundation Establishment Realm, which is why Su Yang could control him.
