We all are aware of the heart present in our physical body whose main function is  to circulate the purified blood. Apart from that, a subtle heart is located in the center of our chest which carries the feelings. It is called 4th chakra as per yogic terms. The more we are connected to God through chanting or meditating on Him, stronger this chakra is and greater is its hold on our physical heart. So a devotee expresses gratitude for filling up his subtle heart with the Divine experiences which sustains him against all situational odds. This forms the basis of hypnotherapy too.