Acknowledge this picture from the Net illustrating where the bindu is, the point the tuft of hair arises from..
Subtle fire amidst eyebrows and bindu
Violent shaking upon meditation
Then relaxing into the heart
Shiva refers to three energy centers in this dharana: ajna, eyebrow center; bindu, tuft center and anahata, heart center.
The ajna chakra, eyebrow center, is the controller of all other energy centers in the mind body system. It relates to the Pituitary, the master gland, which controls the functions of all other glands. This is also the highest of chakras in the human system, bindu and sahasrara being energy gateways rather than energy centers.
Ajna means command in Sanskrit, also Will. It is considered the seat of ego, both for positive and negative connotations. When the energy rises to ajna, and gets blocked, it creates Hitlers and other monsters. When the ajna opens, it creates Ramanas and Gandhis.
Ajna is where ida, left energy path way and pingala, the right energy pathway, finally meet with sushumna, the central energy highway. This is where prana , the inhaled breath energy meets the apana, exhaled breath energy, creating what Shiva calls sukshmagi, subtle fire. This is the fire of Self Awareness, burning the false ego, ahamkara, leading to prajna, awareness.
A lotus with two petals symbolizes the Ajna chakra. Many interpretations are possible: light & darkness; masculine & feminine; sun & moon; ego & awareness. I prefer Shiva & Shakti, Purusha & Prakriti.
When Shakti merges with Shiva here at the ajna, what results is the blissful radiance of the thousand petal lotus of the sahasrara. A Quantum Physicist may call this the wormhole leading to another higher universe. Who knows? Who cares? You will be too blissed out to wonder.
Be careful though with anything you do at the ajna. Gentle is the way. I suffered by being aggressive and foolish in trying to awaken the ajna. Our energy centers do not respond to aggression. They respond to gentle persistence coupled with deep mental conviction. Time can never be an issue. It may take a life time to awaken the ajna. Any speeding can lead to blindness, and madness.
Some tantric scriptures say that the kumkum, a mixture of turmeric, lime and other exotic stuff Hindu women wear at the ajna, stimulates the ajna. Perhaps you can apply kumkum at the ajna, and sandalwood paste at the bindu and anahata, when you try this dharana.
Gently massage the ajna and then focus on it as if you are breathing through it. Do not indulge in any other rritualistic visualization of yantras and repetition of mantras. Keep your awareness alternating between the ajna and bindu. Just be.
You may become agitated, you may become disturbed, mentally and sometimes physically, but as long as all that you do is to be gently aware , you are fine.
I do this while in Yoga Nidra and find it comfortable. There may be a sudden shift of energy within, not unpleasant. I have no idea what it means. I experience and move on.
Gradually, your attention will move to the heart center, the cave of the heart, as Shiva puts it, the seat of inner consciousness. You relax. You dissolve. If this is called sunya or laya, i am ok with it. I am aware and yet i am free of thoughts. I must be thinking because after the experience i do remember what happened.