Grant, Lord God, that we, your servants, may rejoice in unfailing health of mind and body, and, through the glorious intercession of Blessed Mary ever-Virgin, may we be set free from present sorrow and come to enjoy eternal happiness. Through our Lord Jesus Christ, your Son, who lives and reigns with you in the unity of the Holy Spirit, God, for ever and ever. - Collect Prayer
The main difference between the devotions to the hearts of Jesus and Mary is that the one concerned with Jesus emphasizes his divine heart as being full of love for mankind, but with this love for the most part being ignored or rejected, while devotion to Mary's heart is essentially concerned with the love that her heart has for Jesus, for God.
Therefore, it is not an end in itself, so the love of her heart is meant to be a model for the way we should love God. The fact that her heart is immaculate, that is sinless, means that she is the only fully human person who is able to really love God in the way that he should be loved.
Honoring Mary's Immaculate Heart is really just another way of honoring Mary as the person who was chosen to be the Mother of God, recognizing her extraordinary holiness and the immense love she bestowed on Jesus as his mother, the person who was called to share in and co-operate in his redemptive sufferings.
The aim of the devotion is to unite mankind to God through Mary's heart, and this process involves the ideas of consecration and reparation. A person is consecrated to Mary's Immaculate Heart as a way of being completely devoted to God. This involves a total gift of self, something only ultimately possible with reference to God; but Mary is our intermediary in this process of consecration.
Because of the strong analogy between Jesus and Mary, the consecration to Mary's Immaculate Heart is closely linked to the consecration to Jesus' Sacred Heart, although it is subordinate and dependent on it. That is, although the act of consecration is ultimately addressed to God, it is an act that is made through Mary.