Another practice is to pray, in the spirit of penance, with one’s arms outstretched in the form of the cross, in union with Christ crucified, or to pray prostrate with one’s forehead touching the ground, thus abasing ourselves before God Whom we have dared to offend, we who are nothing to his presence. Although such penances are not obligatory, they are necessary in many cases; for example, to help overcome fiery natures which cause people to sin, or the violent temptations of the world, the devil, pride and the flesh.

Quotes from Sr. Lucia's Calls regarding the Message of Fatima (p. 109) - with minor edits in some instances.