Most of us have heard the admonition, "Watch your thoughts". It's almost like Yoga101 or Meditation101. Such a simple practice, surely just for beginners and those starting on the path of single pointedness or concentration, right?
Any of these practices (sadhana) taken to the extreme, have the capacity to take an apparent individual "all the way". The reason why spiritual aspirants make little progress is chiefly because they drop one practice in favour of another, or don't stick with a simple practice until.. and beyond until.
Watching or observing thoughts not only introduces the watcher to the realisation that there is a subject/object set-up happening with thoughts (and the subject is totally identified with thought or feeling-thought), but also that ultimately the watcher itself (you and me) is yet another thought!
Nothing can be said ultimately about any practice, since the aim of a practice is to take one beyond the mind, and thus beyond intellectual description and understanding.
One key is enough. Keep going until the (imaginary) door we are trying to unlock opens of its own accord.
Here is an excellent tip from Krishnamurti on the topic:
"Watch what is happening inside you, do not think, but just watch, do not move your eye-balls, just keep them very, very quiet, because there is nothing to see now, you have seen all the things around you, now you are seeing what is happening inside your mind, and to see what is happening inside your mind, you have to be very quiet inside. And when you do this, do you know what happens to you? You become very sensitive, you become very alert to things outside and inside. Then you find out that the outside is the inside, then you find out that the observer is the observed."
- Pg 36, K on education.