The Chinese Master Wu Wen […] went to Master Huai Shi, who taught him to meditate with the help of the sacred mantra WU. This mantra is sung mentally with the letter U repeated twice: UUUUUU, UUUUUU, prolonging the vocal sound, as if imitating the sound of a hurricane when it howls through the mountain gorge, or like the terrible pounding of the waves against the beach.
The chanting of this mantra is done mentally when we practice meditation with the purpose of reaching the quietude and silence of the mind, when we need to empty the mind of all kinds of thoughts, desires, memories, worries, etc.
Wu Wen sang the mantra WU. He imitated with the U the sound of the wind in the mountain gorge, the sound of the sea when it hits the beach. Wu Wen knew how to intelligently combine meditation with sleep.
Wu Wen sang his mantra with his mind and thought of nothing. When a wish or memory or thought arose in his understanding, Wu Wen did not reject it: he studied it, analyzed it, understood it on all levels of the mind and then forgot it radically, totally or definitively.
Wu Wen sang his mantra continuously. Nothing was desired, nothing was reasoned about, any desire or thought that arose in the mind was properly comprehended and then forgotten. The chanting of the mantra was not interrupted, the mosquitoes and their stings no longer mattered.
Samael Aun Weor
The Buddha’s Necklace, chapter 18