Venerable ajahn Khao’s life and practice is a truly amazing story. It clearly illustrates the Dhamma teaching about the effectiveness of stubborn determination and anger when directed against one’s own kilesas. He was strongly imbued with both of these factors in his practice of Dhamma. Ajahn Khao possessed a remarkably resolute character and liked to put his whole strength into whatever he did. He had been like that since he was a lay person, and when he was ordained he carried these characteristics over with him. The longer he was ordained in Buddhism, which is a true religion that teaches people to act truly in whatever they do, the more he felt impressed by the principles of Dhamma.
Both Ajahn Khao’s mode of practice and his level of spiritual attainment are worthy of the utmost respect. He always preferred to practice in remote, secluded locations with such single-minded resolve that his diligence in this respect was unrivaled among his peers in the circle of Dhutaṅga Bhikkhus.
Ajahn Khao had a streak of very strong determination in his character, which may be seen from what has been written about him. He had no difficulty in sitting in meditation from dusk until dawn – he could sit all night whenever he chose to do so. Sitting in meditation practice from dusk to dawn is no small matter. Unless one is the kind of person who has a heart so full of courageous determination that it could cut through a diamond, one cannot do it. So we should give him our heartfelt praise and admiration. In ways such as this he was fully capable of being an inspiration to his disciples, enabling them to gain peace and happiness by following faithfully in his footsteps. When he was alive and still possessed the five khandhas, he had absolute certainty in himself that he had reached the end of becoming and birth – this was completely self-evident to him. When it came time for him to let go of the khandhas, he attained the state of ultimate happiness (paramaṁ sukhaṁ) in all respects, totally free of all responsibilities and concerns.
Reference: Ven. Ācariya Mahā Boowa Nanasampanno : ‘Venerable Ajaan Khao Analayo: a True Spiritual Warrior’, pp. 208-9.