Vijnana Bhairava 115 Awakening Through Dizziness

 The practice of approaching outside void to awake inner void : 

kūpādike mahāgarte sthitvopari nirīkṣaṇāt |

avikalpamateḥ samyak sadyas cittalayaḥ sphuṭam || 115 ||

"Standing over a well for instance

or a deep precipice, with eyes unblinking,

because attention is undivided,

the mind disappears truly, clearly and at once."