Under the lineage of the great masters...

In "Living Dharma", from Jack Kornfield, three prominent meditation teachers, Ajahn Maha Bua, Ajahn Dhammadaro (not Ajahn Lee Dhammadaro), and Sayaggyi U Ba Khin speak about the importance of the solar plexus.

Let us start with renown Thai teacher, Ajahn Maha Bua (1913-2011):

(please, do notice how, here below, the teacher advocates the optional use of a mental note while observing the breath, to be able to feel sensations clearly and quickly right at the solar plexus. This teaching is extremely valuable, and many practitioners have repeatedly confirmed the efficiency of such a brilliant tactic.

One will never emphasize enough the value of such a suggestion!!!) 

Says Ajahn Maha Bua:

 "In becoming aware of breathing, one should at first fix attention on the feeling of the breath at the nose or the palate (roof of mouth),  as it suits one, because this is where the breath initially makes contact, and one may use this as a marker point for holding one's attention. 

Having done this until one has become skilled, and the in and out breathing becomes finer and finer, one will progressively come to know and understand the nature of the contact of in and out breathing, until it seems that the breathing is located either in the middle of the chest or the solar plexus.

After this, one must just fix one's attention on breathing at that place and one must no longer be concerned about fixing attention on the breathing at the tip of the nose or the palate, nor about following it in and out with awareness.

In fixing attention on the breath one may also repeat Buddho (or rise-fall) in time with the breath as a preparatory repetition to supervise the in and out breathing, in order to assist the one who knows and to make the one who knows clear with regard to the breath. Then the breath will appear more and more clearly to the heart.

After having become skilled with the breath, every time one attends to the breathing process, one should fix attention at the point in the middle of the chest or the solar plexus.

The breath is seen (or felt) in the middle of the chest or the solar plexus, much as it is felt at the tip of the nose in the earlier stages of the practice."

(On being questioned, the teacher said that the middle of the chest and the solar plexus were one place located at the bottom end of the breast bone. But he also said that if one understood them to be two separate places, either of which could be the location for awareness of breathing, one would not be wrong.)

A prominent disciple of Luang ta, Ajahn Pannya makes here a crucial statement, explaining why it is so important to focus on the solar plexus.
Nevertheless, it is highly regrettable that he does not coin the term “bodily sensations”.

That was S.N.Goenka’s approach:
a feeling starts always at the level of bodily sensations…

Ajahn Pannya: how not to generate new karma… 

(very similar to Ajahn. Dhammadaro’s teaching).

Speaking about the crucial link that shows how sensations (vedana) lead to liking or disliking  (tanha) he says:

"...This is an important point, because exactly at the juncture you can place a barrier between the feeling and tanha. 

Feeling is just feeling. It is a result of past kamma.

Whereas the tanha (greed) brings new kamma, so by allowing the two to link (vedana paccaya tanha), you are creating more kamma.

If one can turn away at that juncture and prevent that new kamma from occurring, the situation gradually gets better.   

But the "Paticca-samupada" (pronounce "patit-cha samupada", the chapter about dependent origination) does not give practical guidance on how to do that.

It tells you what happens, but it does not indicate the way to deal with it.

In order to deal with it, you must learn to find out where the feeling is located. Then, you can examine this feeling and see how it gives rise to tanha.

For example, you hear a sound you don’t like. Then based on that sensation an unpleasant feeling arises.

That unpleasant feeling is a feeling of hate. But the feeling of hate isn’t the emotion of hate. There’s no hatred in the feeling. It becomes the emotion we call hatred when you start thinking, criticizing, blaming, and so on. That’s when the trouble comes, in the thinking..."

The feeling of hate is an unpleasant feeling. It usually occurs as a tension felt in the solar plexus area. 

You’ve got to learn to turn inwards and look at that feeling. (that bodily sensation)

That’s very important. If you can turn inwards and look at it, you don’t let it escape and turn into bad thoughts.

So you defeat the kilesas at that point, because no more kilesas are then created by kamma.

Mind you, you only defeat it a little bit at a time. To get rid of hatred completely, you’ll probably have to do it many times, until it gradually, gradually goes.

You’ve got to learn to look carefully inside. Find out where those feelings are located, and what types of feeling arise.

When unpleasant feelings arise, examine them at the point where they occur.

Don’t let those feelings turn into thoughts about blaming others.

Think only about the feeling, not about other people.

In other words, if you keep it internally, no harm is done. Trouble comes only when you let it escape out into thought, speech, action.

And that’s where the kamma is generated.

(then he talks about Nibbana, apparently from his own experience…)

Attaining the stage of Sotãpanna ensures that the mind will no longer deteriorate below a certain level.

We can say, as far as mental development goes, that when the mind attains any of the paths, it has known Nibbãna for a moment.

Attaining the stage of Sotãpanna, it would seem that what happens is that the mind knows Nibbãna temporarily, but the kilesas are too strong for it to stay there, so it is pulled away again. Yet, that knowledge is there and it will never deteriorate below that point.

It is as though the mind goes into an emptiness, an emptiness of all this that we know.

Because there is nothing like it in the relative, conventional world, you just can’t say anything about it.

When you reach that level, then that’s where you are meant to be, that’s all one can say.

I would say that Nibbãna is where we all should be. It’s as though the fundamental ignorance we have in us makes us go in the way of the world and become attach to worldly things, which causes us to do all sorts of peculiar things. That is the wrong way. That attachment pulls us away and prevents us going in the direction of Nibbãna.

If we could just drop everything, BANG, we’d be there.

Another chapter of “Living Dharma” is devoted to the method of Ajahn Dhammadharo (1914-2005), not to be confused with Ajahn Lee Dhammadharo…

"all our senses, eye, ear, nose, tongue, touch and mind, arise in conjunction with the six sense objects. When consciousness arises but mindfulness is absent, the knowing of objects through the senses leads to liking or disliking. This leads to craving for continued pleasure, and further to clinging, action and rebirth." "mindfulness of sensation will lead us to experience all the senses directly at the heart base (in this tradition the heart is known as the seat of mind).

When all experience, including thinking, becomes perceived as clear sensations arising and passing away at the heart base, we directly and deeply experience the truth of impermanence, suffering and non-self.

This leads us to the deepest truth of all, the end of suffering, the experience of nirvana..."

In his teacher's footsteps, Ajahn Thammadipo, a prominent disciple of Ajahm Dhammadaro, makes it very clear, that the exact location of the heart base is the bottom end of the breast bone, i.e. the solar plexus. (see picture below, how he points to the crucial body location)

Later on, Ajahn Thammatipo, managed to try more movements, but keeping the same goal in sight, that is to feel a clear sensation below the breast bone, that he says to be the seat in the mind.
We see him in the below short clip showing one of these movements.