srog shing

སྲོག་ཤིང
1) main beam, axle; 2) chief/ essential part, nucleus, core, backbone, what holds something together central pillar life tree/ pillar [IW]

Life-tree, placed as an axis at the center of a stupa or a statue [RY]

central axis; mainstay [RB]

the life tree, the life pillar [inside a stupa or statue] [RY]

axle, life-tree, axle-tree, life-trunk, stick in center of statue to indicate the central channel, column of life, support for vital principle [JV]

axis, axle [RY]

Also:

Drak Yerpa (brag yer pa) - This is the holy place of Guru Padmasambhava related to the speech aspect. It is also said to be the "life tree" (srog shing), or spiritual axis, of Lhasa. There are over eighty caves where many great beings from all lineages meditated. On the top are the cave of Guru Padmasambhava, the Rock that is Hard to Reach (brag gi yang bgrod dka'), and Yeshe Tsogyal's Secret Cave (gsang phug). Below is the Moon Cave (zla ba phug), another cave of Guru Padmasambhava; the Dharma King's Cave (chos rgyal phug), King Songtsen Gampo's cave; the great cave where the eighty siddhas of Yerpa (i.e., Guru Padmasambhava's disciples) meditated together; and Lord Atisha's cave, the Cave of Auspicious Coincidence (rten 'brel phug), thus named because when Atisha entered it, a rain of flowers fell. See Gegyepa Tendzin Dorje (1988) and Keith Dowman (1988, hereafter abbreviated as PP). In some enumerations, instead of Drak Yangdzong (see chap.10, pg. 272, note 59, in The Life of Shabkar), Drak Yerpa is given as the holy place related to the body aspect of Guru Rinpoche. (MR-ShabkarNotes)