Heidegger’s “Turn” (Kehre) was less a turn down a new track than a glance forward over the shoulder, as he continued to walk backward into the future.
The early academic and later “mystic” philosophies are both hermeneutic readings of the philosophical canon, albeit with the addition later of “philosophical” poets like Hölderlin, Rilke and Trakl, and phenomenological, that is, concerned with how being or beings appear, though there is a move from talking about a (transcendent) Being of beings as the “ground” of beings to talking later about an (immanent) “happening” (Ereignis) of being.
However, the later philosophy is more explicitly prescriptive; it develops the early critique of the “shallow,” “uprooted,” etc. contemporary way of being—or “dwelling,” to use the later term—to describe aspects of this way of being like technology, the world picture, art/architecture, etc. in more depth, and offer prescriptions for how philosophy and art/architecture can help us to dwell better. The two (really one-and-a-half) philosophies might be defined thus:
- early: description of how things are, assuming that the ontological (essential, a.k.a. existential) “precedes,” i.e., is more fundamental than, the ontic (everyday, a.k.a. existentiell), though we normally disregard the ontological;
- later: prescription of how things ought to be, assuming that the ontological is better than the ontic, i.e. we ought to be a certain way in the world to be well.
The early philosophy is heavily indebted, then, to the tradition of Platonist misreaders of Kant—in particular, the Idealists: Fichte, Schelling, Schopenhauer, etc.—who tried to say how we might transcend the world that is empirically or cognitively available to us, usually through some form of direct intuition (intellectual, artistic/mystical, or voluntary [of the will], respectively), and what we might meet there (i.e., the so-called thing-in-itself, or what Heidegger called the Being of beings).
Camille Flammarion. [“A missionary of the Middle Ages tells that he had found the point where the sky and the Earth touched. . . .”] In L’Atmosphère: Météorologie Populaire (1888)
Like an anxious Flammarion who refuses to leave his body behind, Heidegger finds nothing beyond the horizon: things are grounded in nothingness (see “What Is Metaphysics?” —and his admission in “Existence and Being”  that the position outlined there was only a step on the way and not an arrival).
Paul Cézanne, Le Jardinier Vallier [The Gardener Vallier] (1906)
After the Turn, Heidegger turns to art. He thinks of being and beings as folded into each other, an in-folding (or immanence of being with beings) that is revealed in various “mystic” experiences (see “The Fieldpath” ), but most cogently in art like Cézanne’s, in which the twofold [Zweifalt] of Being and beings (of presence and what is present, a.k.a. the “ontological difference”) is overcome in the radiant oneness [Einfalt] of the artwork:
In the late work of the painter the twofoldness / of what is present and of presence has become / one, “realized,” and overcome at the same time, / transformed into a mystery-filled identity. / Is a path revealed here, which leads to / a belonging-together of poetry and thought [ein Zusammengehören des Dichtens und des Denkens]?
(“Cézanne” [1971; rev. 1974], Denkerfahrungen 1910-1976 [Klostermann, 1983] 163; see Julian Young’s brief excursus). In full:
“Good” art (art as poetic thinking, perhaps) is futurological: it embodies a kind of watchful waiting (Gelassenheit, lit. “calmness, composure”; for Heidegger, “letting [beings] be“) that constitutes a “right” attitude to things (a.k.a. “releasement toward things” in “Conversation on a Country Path” [1944-45; in Gelassenheit [Günther Neske, 1959], trans. as Discourse on Thinking [Harper, 1966]). In prescribing such a course of action, Heidegger, as thinkers are wont to do, casts an errant glance forward over his shoulder.