Of Dominic's ways of prayer, the section on physical asceticism is described with succinct discretion. In our aggressive and violent culture, great discretion is called for in discussing this for us too. People sometimes read about this kind of prayer and get turned off altogether or else they engage in very self-destructive behavior. So, I hope the reader will forgive the length of this post this time.
It is true that many of the saints sometimes engaged in self-destructive behavior when they were beginning the spiritual life. Part of what they were dealing with was their own broken instinct of self-preservation, an affliction common to all humanity. Self-preservation is sewn into human nature so that we can protect ourselves from legitimate danger. But the saints well understood that this instinct sometimes inclines us away from things that put this life at risk even when our eternal life is on the line. So, they acted against this propensity - sometimes in excess. In most cases, we can find witnesses or their personal writings where they repented and expressed regret for their actions. They came to see that physical asceticism must always be ordered to human maturity.
Indeed, the Fathers of the Church understood that the glory of God is man fully alive. Human maturity, the fullness of life, is realized when we are free to enter into communion with one another and God. It is to this end that the instinct of self-preservation must be ordered or it will continually cause problems for the spiritual life.
It is from the context the this particular practice of St. Dominic should be considered. Witnesses recalled that St. Dominic would beat himself with a chain while praying Psalm 17 (18) -- a psalm of deliverance from death and war against the enemies of God. This austere practice was meant to be an expression of compunction.
In other words, while praying for deliverance with his lips, St. Dominic was also acknowledging that he was responsible for the terrible plight he was in and, further, he was somehow responsible for the terrible plight others were suffering. The context of this prayer was war. His body was involved in spiritual violence. The discipline he took on expressed an interior conflict. This extreme form of petition addressed the inclination to forget things that the "self," the big fat ego, would rather avoid. He wanted to keep in mind his responsibility for his own sin and for the evil suffered by others - because it is by accepting the truth that God can begin to act.
When we are not mindful of the consequences of our own actions or lack of action, we lack the humility, the truthfulness of heart that God searches for in us as He hears our prayers. Without this truthfulness, even our natural instinct for self-preservation can become self-destructive - like when we are callous towards someone who has hurt us, or we have hurt, because on some level we reckon it too painful to deal with them in any other way. Insofar as these hidden motives are driving us, we are not free to love, to follow the prompting of God to move beyond "self." The tradition sees the need to reorder our instinct for self-preservation in Christ's teaching: "Whosoever wants to preserve his life will lose it. (Matt. 16:25)"
Is there another way to such truthfulness - the only kind of truthfulness that can heal our instinct for self-preservation, ordering it to eternal life? Therese of Lisieux, our newest doctor of the Church, did not think she could take up the discipline with the same heroic resolve of the great saints. But she wanted to be a saint. She remembered during a pilgrimage to Rome how instead of climbing stairs she took an elevator - and how much easier the elevator was. She believed that the Church needed to develop a spiritual elevator so that people like herselve could more easily become saints. This is what she proposes in her Little Way.
Rather than the physical mortification saints embraced in the past, she advocated interior asceticism. This practice involved among other things acting against our instinct for self-preservation in social situations. Accordingly, when she was misunderstood or falsely accused, instead of mounting her defense or lashing out in righteous indignation, she would offer the situation to God and thank Him that she was deemed worthy to be treated like Christ - then smile or at least silently walk away. In other words, instead of a physical discipline, she used the effort to love her persecutors to be her mortification.
In doing so, she recaptured an important biblical principle often lost on those who are overly concerned with exterior practices: following Christ involves a radical renunciation of the evil in my own heart, a mortification of every impulse which is not for the glory of God. As St. Paul says - the old man needs to be put to death. There is a constant pull to our former way of life, to follow the sinful patterns of life of our ancestors, a way of life that frantically grasps at preservation of one's self. Instead of going back to the past- Christ challenges us to imitate Him.
Renouncing the tendency to always preserve and promote oneself is what it means to pick up our Cross and follow the Lord. The suffering that must be endured until this renunciation is complete is part of the price of discipleship. This is essential to the interiorization of morality we see in Matthew 5. It speaks to a transformation of the inner man - making us capable of real communion with God and one another.
In conclusion, prayerful asceticism is an important part of the spiritual life because our instinct of self-preservation is broken and needs to be healed by the Lord - re-ordered for this glory. However we pray through this kind of instinctual brokenness, we must keep our eyes on the communion the Lord establishes us in and realize that interior transformation is God's work. Our efforts then aim at making space for the Lord to act - this is why Therese's Little Way can be even more mortifying than a physical discipline.