Recently I came across a progressive series of workshops working out of the Kusum Rangshar (by Shardza Tashi Gyaltsen) where one of the classes revolved around a practice called Thögal (ཐོད་རྒལ་).
Searching for this term to determine what exactly it is has not been particularly fruitful in English-language sources other than to say that it is linked with the practice of dzogchen, so I am wondering if there is a reasonable high-level description of what it is or what it entails?
While I'm clearly out of my depth with this question, let me give you a few pointers.
(TL;DR version: Thogal is cultivation of "suchness", aka tathata).
First, you need to understand what Dzogchen, or Great Perfection, really refers to. Basically, this is the direct realization of the Heart Sutra experience. In other words, you need to clearly see that what you imagined to be the solution, is really a part of the problem. To quote Dogen,
The water is clean, right down to the ground,
Fishes are swimming like fishes.
The sky is wide, clear through to the heavens,
And birds are flying like birds.
Second, you need to have experienced a glimpse of your "true self" or "true nature". This is separate from the above, and requires a teacher to push you off "the cliff of your ego", one way or another.
Then, once you've seen your true nature both ways, you will have no doubts as to why you want to cultivate it, neither at conceptual, nor at experiential level.
In my limited understanding, Trekcho ("cutting through") and Thogal ("transcending the peak") are two aspects of cultivation of the true nature. Trekcho is fearlessly cutting through all your B.S., while Thogal is learning to spread the wings of natural spontaneity, with no sense of observer, no boundaries -- the space is dancing by itself.
To give a totally unrelated quote (at risk of getting killed by lightning for messing with the will of gods LOL!)
Dance like there's nobody watching,
Love like you'll never be hurt,
Sing like there's nobody listening,
And live like it's heaven on earth.
Or to quote Chogyam Trungpa,
[Trekcho]
You need to develop a state of mind that is cutting through fundamental spiritual materialism. Beyond this, there is nothing to cut through.
...
At the same time, we see that destroying spiritual materialism is itself a form of spiritual materialism -- and destroying that destroying is spiritual materialism as well. Finally we begin to see that even the idea of cutting through becomes questionable.[Thogal]
The mandala practice of Dzogchen is to just experience things as they are, fully and completely. You realize that there is nothing to hold on to, and at the same time, there is no way to make the groundlessness another ground.
...
You experience the vastness of space and the sense of no boundary. You experience the playful aspect of space, in which there is no hope or fear, pleasure or pain, which is the best dance. And you experience the pragmatic aspect of space: the fact that space is hopeless, and it does not mean anything very much.
...
This discipline of noncaring is not so much the accomplishment of the path, but rather the style of the path.
As you can see, in all truth, I have no idea what I'm talking about. But here are a couple of links that may shed more light on this obscure topic:
In terms of practice, the key point I've learned is that thögal can only be practiced from the perspective of rigpa (pristine awareness); which in turn assumes that one has achieved shamatha, perfected vipashyana, and completed trekchö. Even kind, authentic teachers that openly and lucidly teach Dzogchen do not generally teach thögal, simply because there is absolutely no benefit in doing so until the student has truly realized rigpa. In the meantime, pretending to practice it is a distracting fantasy at best.
(Bear in mind that I have no knowledge of Dzogchen, this is only what I found through search engines.)
Tögal (Tib. ཐོད་རྒལ་, Wyl. thod rgal) — one of the two aspects, along with trekchö, of Dzogchen practice.
Tögal, translated as ‘direct crossing’, ‘the direct approach’ or ‘leapover’, can bring very quickly the actual realization of the three kayas in this lifetime, and thus is a more rapid way of bringing about the dissolution of the practitioner’s karmic vision. The practice of tögal brings the realization of ‘spontaneous presence’ (Tib. ལྷུན་གྲུབ་, lhundrup), and it can only be undertaken by a practitioner who has first gained stability in the practice of kadak trekchö.
Sogyal Rinpoche writes:
Only when the master has determined that you have a thorough grounding in the practice of trekchö will he or she introduce you to the advanced practice of tögal. The tögal practitioner works directly with the clear light that dwells inherently, “spontaneously present,” within all phenomena, using specific and exceptionally powerful exercises to reveal it within himself or herself.
Tögal has a quality of instantaneousness, of immediate realization. Instead of traveling over a range of mountains to reach a distant peak, the tögal approach would be to leap there in one bound. The effect of tögal is to enable a person to actualize all the different aspects of enlightenment within themselves in one lifetime. Therefore it is regarded as the extraordinary, unique method of Dzogchen; whereas trekchö is its wisdom, tögal is its skilful means. It requires enormous discipline, and is generally practiced in a retreat environment.
Yet it cannot be stressed too often that the path of Dzogchen can only be followed under the direct guidance of a qualified master.
-Tögal @ Rigpawiki.org
While I believe that the other answers here are more on-point from the standpoint of the question, after I learned more about it (though I won't claim to understand it) I wanted to add one additional piece more from the mechanics side than the inherent nature:
Thögal is a practice of cultivating what are called "clear light" visions based on top of the natural state of mind. There are several components which are designed to facilitate and cultivate the vision aspect, but it requires first as a prerequisite a steady natural state of mind.
In his teachings around the Heart Drops of Dharmakaya (by Shardza Tashi Gyaltsen) Khenpo Tenpa Yundrung Rinpoche said the following (transcribed):
The Tibetan word Thögal has many meanings. The most known is Thögal as a practice that renders manifest the visions of Clear Light. In some contexts, the term Thögal actually refers to the Clear Light itself.
[...]
When things are presented in this way, it seems that the Thögal practice is more exalted than the Trekchö practice, as if the Thögal is better than the Trekchö. But without Trekchö there is no possibility of practicing Thögal. Therefore Trekchö is the most important practice, and as such is the core of the Dzogchen teachings.
