This was an auxiliary martial skill that could directly distinguish the strength level of the other side's cultivator. Even if the other side didn't use the mantra, it could also be seen. This martial skill was actually of little value. One could use the other side's strength level to raise one's own martial skill to the same level as the other side's strength level to attack!
It was a heaven-defying practitioner's art that borrowed strength to counter strength, but now that Xiaowu was a Sky Class late phase Peak Master, she could only use Sky Class late phase peak martial skills. Unless she met a master in the great circle stage, she wouldn't need to use this at all.
Of course, before he had his current strength, his auxiliary martial skill was still quite useful. He used to be a very powerful cultivator. When he was in the earth-class, he could also exchange a blow with a high-level cultivator and escape. But now, there was nothing special about it.