Other than good ol' blood, sweat, and tears on the mat, I have learned the little bit I know about tristana from study of books on Ashtanga and, to a lesser extent, study with Western Ashtanga teachers. In something like 15 total weeks spent with Sharath, to my knowledge I have not heard him discuss it, but this isn't much of a surprise. He has his hands full; at any given time at least 50% (usually more) of the students at the shala seem to be relative beginners in the sense that they are working on Primary and possibly the first third of Intermediate. Remember that additional techniques (eg Ashtanga pranayama) are not taught until at least the completion of Advanced A, so I usually consider that students are beginners until at least that point.
Tristana means "three points of attention" or, maybe "the union of the three points of attention." "Yoga" can mean a technique or a state, but usually means a technique; "tristana" is the same, but usually means a state, a state of flow attained when everything falls into place just right. However, it can also more pragmatically be used to discuss incorporating the various strands of Ashtanga practice.
The three points of attention are vinyasa, bandha, and drishti. In effect, this is at least four points of attention, as vinyasa includes ujjayi breath + correct movement in and out of asanas. Bandhas are the physio-energetic points of attention along the central axis of the body, just above the perineum and in the lower part of the abdomen. Drishti is a specified gazing point for each movement. So, for every moment of Ashtanga practice, there is a specific breath, body position, internal focus, and gazing point. When, through repetition, these individual, effortful elements of experience coalesce into an effortless, absorbing state, it is called tristana. Tristana, in my humble opinion, is what gives Ashtanga Vinyasa yoga such depth and challenge.
Some historians of contemplative traditions point to a distinct shift in religious ceremony in Indian history. Ancient shamanic cultures believed that their ceremonies drove the universe, and therefore invested time and energy into performing rites properly to ensure the crops would grown, the seasons would follow one another, the rains would come, etc. Over time, these ceremonies become more and more intricate and at some point, the priests began to realize that perhaps the only element of the ceremony not yet under control was the state of mind of the priest. Thus contemplation entered spiritual practice and ceremony.
Similarly, each individual passes through most of the stages of his or her species and culture, in condensed form, during the lifetime. There is a point during Ashtanga Vinyasa practice when one realizes that in attempting to manage all of the separate pieces of breath, posture, count, gaze, etc the most important (and slippery) element is the mind and, in fact, it is the influence of the mind that all of those separate elements aim at anyway.
There is a paradox in that moment, for it becomes clear that the thinking mind can in fact only focus on one thing at a time. Like a smartphone it can keep several apps running, but only one at a time can really, truly be at the forefront at a time. The only way for these elements to come together and be held equally in attention is by weaving them together in an absorbing experience, one that goes beyond simply "thinking about" something. This act transcends the thinking mind.