'The solidarity of the shaken' is primarily discussed in his Heretical Essays in the Philosophy of History, published as samizdat in 1975. The work, which is quite dark, is a reflection in six essays on nihilism and war, and could be regarded as an extending of the Heraclitean saying, "War (Polemos) is father of all." We find ourselves going about our lives in the comfort of the familiar, the habitual everyday, and this prereflective attitude is one that becomes severely disrupted by major events. In the face of a truly major event, one such that we can no longer trust to the familiar categories and ordinary ways, we are stimulated to a higher reflection -- our assumptions about reality are ripped by reality itself, we are shaken from our everyday, and we are forced to find a way to continue in the face of this shaking. Patočka posits this as a significant driver of history. Polemos is the father of all; polemos gives us the polis and philosophy. In the Persian and Peloponnesian Wars, for instance, Athens undergoes a transformation, and the transformation is that of an awareness of itself as a city, and a city of a certain kind.
Perhaps the most important of Patočka's examples are taken from the twentieth century, which is in a very preeminent way a century of war. And Patočka points to an experience not uncommonly had in the trenches on the front, of all the assumed meanings of the world vanishing away, but this being experienced not merely as a loss but also as a liberation. You are faced with the fact that you were sent there as a sacrifice but this also brings to the fore a sense of one's freedom for self-sacrifice, not for this or that particular cause, but as such, as a free and living being; and one also is faced with something so immense that it puts everything in a new perspective -- the enemy is not merely the enemy, but someone enduring the same hellish situation. It is a situation in which one might expect nihilism but instead in which openness to this reality becomes a new kind of meaning, and a result of it is the solidarity of the shaken, "the solidarity of those who are capable of understanding what life and death are all about, and so what history is about" (p. 134). This solidarity is not so much deliberately formed as called forth by the experience of the shaking itself; it is a sense of 'we are all in this together', where this is something terrible -- but also terrible enough to bring a sort of clarity of mind with it.
Everydayness is what leads to war; we go about our plans for peace, and strife grows from the conflict of plans that we are assuming. The solidarity of the shaken raises us out of that:
The solidarity of the shaken is built up in persecution and uncertainty: that is its front line, quiet, without fanfare or sensation even there where this aspect of the ruling Force seeks to seize it. It does not fear being unpopular but rather seeks it out and calls out quietly, wordlessly. Humankind will not attain peace by devoting and surrendering itself to the criteria of everydayness and of its promises. All who betray this solidarity must realize that they are sustaining war and are the parasites on the sidelines who live off the blood of others. (p. 135)
This solidarity, in other words, can be communicated; the shaking is not ineffable. But notice also that the solidarity can be betrayed. There is a sense in which this solidarity is fragile. It is hard for us all to be in this together if this ceases to be; we can slip easily back into the everyday. This is an aspect Scruton explores in his novel at some length.
Jan Patočka, Heretical Essays in the Philosophy of History, Kohák, tr., Dodd, ed., Open Court (Chicago, IL: 1996).