This was such that once one cultivated them, Yue Dongliu would sense it.
Take for example, the Full Moon Breathing Technique. If Su Jingxing cultivated it, the Bloodseeker Beast that Yue Dongliu resided in or after he successfully possessed a human body, he would be able to sense Su Jingxing's location even from thousands of miles away.
Furthermore, once he successfully cultivated the Full Moon Breathing Technique, Yue Dongliu could still silently influence Su Jingxing through the secret door he left behind, change his behavior, and do some unconscious things.
Of course, most importantly, if Su Jingxing really cultivated the Full Moon Breathing Technique and made achievements, then Su Jingxing would be Yue Dongliu's best target to possess!
All of this was part of Yue Dongliu's plans.
However, the genuine version of the Full Moon Breathing Technique that Su Jingxing read from his fading memories, he did not find these drawbacks.