Yan Yu was immune to the attacks of this water-based Taoism Method, and the reason was actually multifaceted.
The most important, of course, was the mutual conquest of the Five Elements, with earth conquering water. Coupled with the Barrier Charm's ability to resist knives and fire, it was an expected outcome that he remained unharmed.
But while he sustained no injuries, why wasn't he swept away by the water?
That's because at the very moment the flood appeared, he immediately activated the Curved Curse and thrust his Immortal Sword forcefully into the ground below.
So the most advanced methods of attack often just need the simplest way to break them... The key is whether the experience and awareness can keep up.
Yan Yu studied the meditation cushion for a long time before he discovered that there was indeed no mystery hidden within it.
The real mystery was pressed under the cushion, a delicate jade hairpin.