how important or significant in the context of nyingma schools is Terdag Lingpa's cycle Rigdzin Thugthig (The Heart Essence of the Vidyadhara)? Thank you.
Terdak Lingpa was a very important Nyingma master and terton and his work have had a huge impact on the Nyingma tradition and in particular the Mindroling tradition. http://mindrolling.org/history/TerdagLingpa.cfm
I don't personally know the Rigdzin Thugthig but obviously Khandro Rinpoche considers it very important, that is good enough for me.http://lotusgardens.org/programsandretreats/AR-2012.cfm
"The direct, hard to understand, subtle field of knowing, the Great Path, is non-conceptual (akalpana), and entirely beyond the grasp of intellectual thought. Divorced from verbal ideation, it is difficult to point out and as difficult to enquire into. It cannot be communicated through words and [therefore] is not within the scope of the neophyte (adikarmika). Nevertheless the path is to be approached through studying scriptures (sutra) of the World-Teacher and following the personal instructions (upadesa) of one's Guru-ji."
Bodhicittabhavana by Acarya Sri Manjusrimitra