To put across how important any breakthrough in understanding here would be, the fact is that if we're not engaging in Satipatthana we're not on the path to enlightenment. Full stop. I think it's just the basic Buddhist meditation. So we have to know what Satipatthana means. Surprising then that someone could discover something radically new about it, and then I could as well? Well, see what you think.
Anuruddha makes the crucial importance of Satipatthana clear to us when he says; ‘whoever neglects it is also neglecting the Noble Path leading to the extinction of suffering while whoever undertakes it has undertaken the noble path leading to the extinction of suffering’ -- Great Disciples of the Buddha: Their Lives, Their Works, Their Legacy (and also in SN 52:2).
The first good explanation of Satipatthana I ever came across was Bodhipaksa's;
"Because the nature of contact is to condition feeling, and the nature of feeling is to condition emotion and thought, and the nature of emotion and thought is to condition action, in cultivating each of the first three satipaṭṭhānas we are in fact cultivating, stage by stage, an awareness of a process. This, I believe, is the true purpose of the first three satipaṭṭhānas. They are not merely a convenient way of slicing up our experience so that we can examine it in a comprehensive way. Rather, they are a way of coming to recognize how we generate action (skillful or unskillful), and the inner climate of experience that leads to our actions." — From his website - http://www.wildmind.org/…/the-four-foundations-of-mindfulne…
I applied this understanding received from Bodhipaksa to my new understanding of relaxation as reducing unnecessary effort. Putting both together I saw not only what we need to do to consciously relax (i.e. investigate the Satipatthanas as a single process) but why Satipatthana was really called The Direct Path. It is called this - and I haven't heard anyone else describe this yet - because when we use mindfulness specifically for reducing unnecessary mental and physical activity this process takes us directly into jhana, and finally leads us to dropping the ultimate unnecessary effort we make in the deepest level of our being; the grasping onto self.
There are further steps after Bodhipaksa's cycle, in the reverse order; we notice how by dropping the unskilled volitions (such as sense craving) our unsatisfying feelings tend to reduce, and that way we walk the path to liberation from dissatisfaction. So we can see the dynamic process in both ways. Awareness of feelings leading to volitions and awareness of volitions leading to feelings. But the second process, volitions leading to feelings, is the really crucial one because this is the process of how we liberate ourselves from dissatisfaction.
Satipatthana was called The Direct Path because it involves reducing unnecessary activity (i.e. relaxation).
A direct path is one which does not make unnecessary detours. Without this reduction element Satipatthana just isn't The Direct Path to enlightenment. Relaxation is crucial to how Satipatthana must work. I suggest the primary importance of mindfulness is that it reveals to us what effort or action is unnecessary, so it can be dropped. This is how we abandon suffering; not primarily through the addition of action (we are talking about something which is ultimately natural).
If you don't believe me then just read about the story of Bahiya's very rapid liberation. If we were to drop all unnecessary effort right now we would instantly become enlightened. It really is a direct path. It can be that direct because enlightenment is not actually any kind of effort to be actioned or maintained.
I'd also better describe what Satipatthana employed for reducing unnecessary effort (i.e. relaxation) looks like in practice.
So through close investigation of our body we find feelings of dissatisfaction. Then we investigate more closely this dissatisfaction and find that it is caused by activity we ourselves are making (mental and physical volition such as the five mental hindrances), all of which we can see as unnecessary activity that is causing us less satisfying experience. So investigating the body we find the feelings of dissatisfaction and investigating the dissatisfaction we find the unnecessary mental and physical activity we are making which hinders us from enjoying jhana or complete liberation.
So this describes how the Four Foundations of Mindfulness are really a process of relaxation. The crucial thing is seeing how we are causing our own dissatisfaction or discomfort and dropping the self-made activity underlying it. When we see directly how we are causing our own dissatisfaction in this way then we can reduce it. This meditative process is experienced as deeply satisfying relaxation. Now if you'll excuse me I'm going to get back to doing some!
I describe this in more detail in my book; The Path of Relaxation.