Zazen a Dzogchen
Is there any "practice" that enhances what is pointed out in Dzogchen, Mahamudra or Zen, that is in itself not a gradualist method or approach?
Yes indeed! The sitting practice of Soto Zen called "shikentaza" is ideal. Shikentaza means "just sitting".
Dogen Zenji described it as the practice "of" a Buddha, not a means to become a Buddha. So there is no expectation of gaining some higher state or enlightenment.
However, by engaging in lengthy sessions of shikentaza the mind becomes crystal clear and transparently vast.
It's like if you have a jar filled with water, mud and sand, and you swirl it around, the water is murky and unclear. If you set the jar down, just leaving it still, the sediment settles to the bottom and the water becomes perfectly clear automatically.
Likewise by leaving the mind alone and undisturbed, it's turbulence will settle down and the mind will become crystal clear on its own. But it's important that no self-effort is exerted nor any expectations be held.
Here's how:
One style is to sit in a relaxed meditation posture on a cushion or on a chair; with back straight. Either look at a point on the floor slightly in front of the body, or look straight out, slightly upwards, and keep the eyes fixed and not moving.
Breath slowly and naturally.
Your mind-state is that of being a mirror; all thoughts and perceptions appear in consciousness but nothing is engaged with. You are simply present and aware with no other agenda.
An alternative practice it to do all the same, but look into the sky without moving your eyes with your mouth slightly open.
The longer you sit in shikentaza the better; the more settled and clear the mind becomes. When it becomes crystal clear and transparently empty, profound wisdom insights will arise spontaneously over time.
This was my core practice since I was 16 when I was a student of Japanese Soto Zen master, Matsuoka Roshi. I started with forty minute sessions in the Zendo, and worked up to two to three hours of unbroken shikentaza. It usually took at least two hours before the wisdom (rigpa) insights began arising spontaneously.
It's especially beneficial to practice in nature if possible or at least in a room that has some good fengshui (aesthetic balance).
(Thogal is also such a practice)