Even if an iron wheel whirls on your head
Perfectly clear samadhi and wisdom are never lost.
You cannot cling to the idea that the mani pearl, or wisdom, has concrete existence. However, if you accept and realize “mani pearl” Dharma, then you will never lose the gem. The iron wheel is an ancient weapon ─ a spinning, red-hot wheel with sharp blades that cuts and sears anything it touches. Once you attain the mani pearl, nothing can deter you in your practice, not even this frightening weapon whirling on your head. Again, the pearl symbolizes perfect samadhi and wisdom.
You can interpret these lines another way: Once you accept Buddhadharma and follow its teachings, you should view your practice as the most precious thing in the world, more precious than life itself. Even if the iron wheel hovers over your head, you should not leave the practice or abandon the Dharma.
— The Sword of Wisdom by Master Sheng-Yen (page 225-226)