Three voids dhyana

Sri Sadgurubhyoh Namah

VBT 45 :

Prishtashoonyam moolashoonyam hrichchoonyam bhaavayet sthiram,

Yugapan nirvikalpatvaan nirvikalpodayas tatah

Translation :

Steady meditation simultaneously on void of back, void of root, and void of heart, leads to state of nirvikalpa (thoughtless-ness).

Commentary : In the previous sutra, it is mentioned to meditate on void of back (sushumna) and void of root (mooladhara) simultaneously to understand the void-mind (silent mind). Here in the sutra, one more void is added, the void of heart (Hridhaya-akasa, the space inside heart). 

How to do :

Heart is made up of emotions. To meditate on heart as void means to consider all emotional thoughts to be void / insignificant / not affecting you. Do not be attached to any of the emotional thoughts as 'mine'. distance yourself from mind. 

Mooladhara (root) is made up of body consciousness. To meditate on root as void means to give up the wrong notion that you are the body. Leave aside the body consciousness even for a moment.

Take your awareness to the middle of sushumna the middle of spine, and meditate on it as if it is made up of space / void.

Do these three void dharana / dhyana steadily for sometime and a state of Nirvikalpa ensues. Nirvikalpa means 'free from thought'.