"The Immaculate Heart of Mary" is in a sense a title of Mary, focusing on her pure love for God and for mankind. Her physical heart is a symbol of her interior life and pure love. This love is twofold: her love of God, her Creator, which we seek to imitate. And her love of mankind, as she prays for us, her spiritual children, without ceasing.
In Catholic theology, Mary always leads us closer to Christ. Trusting in Mary does not take away from our trust in Christ. On the contrary, trusting in Mary will increase our faith and trust in Christ, as Mary takes us by the hand closer to Christ than we ever could on our own. So saying "Immaculate Heart of Mary, I place all my trust in you" could almost be thought of as a "shorthand" for saying: Mary, who love God and mankind with an immaculate and pure love, I place my trust in you, because Christ has given you to us as a secure means of coming closer to him. Something along these lines.
