Unmani means a state of not thinking, and this mudra is useful when doing pranayama, meditation, and kriya practices.
Sit in a comfortable meditative position, with your eyes open and relaxed.
Inhale slowly and fully. Then hold your breath and at the same time focus your awareness at the back of the head. This should last a few seconds.
Then exhale slowly and at the same time taking your awareness down from the back of the head down the back and through the chakras to the base of the spine.
As you are exhaling and moving your awareness down the spine, your eyes should gradually close. When you reach the base of the spine, your eyes should be fully closed.
Inhale slowly and deeply again and begin a new round. Do between 5-8 rounds.