I don't really know if this is a "secret" because it definitely has been proven by people brave enough to choose thankfulness in all circumstances...those individuals are just rare (unfortunately), so maybe that is why this is still somewhat considered a "secret" truth :)  It takes practice to develop the process of turning toward God and intentionally making note of the goodness in the middle of everything that seems hurtful.  It is easier NOT to choose thankfulness because in our selfishness we want people to identify with us (commiserate) or we want to be understood or we want to dwell in that negative experience for a while to feel sorry for our "losses".  I know....when you choose to focus on positive things and to set your heart on God and in Christ...and to talk about or post the things you are thankful for, most people think everything in your life is perfect and that you do not struggle.  In reality, you may have some deep struggles and you may be experiencing a heartache which no one can understand except for you and God. However, you are working really hard to be thankful anyway and you are disciplining yourself to walk in Christ for your strength and to surround yourself in His Word and in His peace for your  heart's protection.  I understand and I am learning why it is important not to judge or be envious of others when they seem to be always thankful or joyful....I am learning why it is wise for me to just love and let God be the judge.  I am learning how far my thoughts can wander into dark and lonely places if I do not choose love, joy, thankfulness, and hope...in Christ.

I don't really think there is a secret to unleashing anything about God, but I understand this statement and the principle of it.  It is scriptural and coincides with many of God's promises to us, but it is just a truth that is hard to live by sometimes.