As one learns about various schools of psychological healing, especially the healing of trauma, it's pretty natural to grope for an intuition for what they hold in common. Hence the question,
What are the conditions under which healing takes place?
My current best understanding takes the general form,
Healing is facilitated by re-experiencing things in a special way.
Wounding past events should first be recalled, even indirectly, and it is then that our response to their presence in our psyche can be reprogrammed. (I'm summoning the recent-ish discovery in psychology that recalling memories rewrites them, and therein lies the opportunity.)
Schools of therapy tend to have their preferred modalities, aspects of experience that serve as focal areas of re-experiencing. For example, somatic styles use bodily sensation, IFS uses theory of mind (perhaps narrative of personality), and CBT uses literal re-enactment. The diversity of possible modalities seems, to me, to be a corollary of how our memories connect holistic pictures of our experiences, rather than recording isolated sensations.
We often have processes of homeostasis that resist simple habituation, and, moreover, unassisted re-experiencing things can lead instead to triggering and retraumatization. So the fact of re-experiencing on its own is not enough: there must be something about how we do it.
As far as I can tell, the various healing methods all have something to do with how we use our attention while we re-experience.
Usually the key is to attending to an intuitive sense: compare IFS's notion of Self, Focusing's felt sense, and Buddhism's sati. Accessing the intuitive sense effectively is usually requires a state of presence or mindfulness, wherein other processes no longer obscure the way. Our active search for the sense must be done with curiosity, an eager receptiveness to accepting it whatever form it takes. A modality uses its avenue of re-experiencing as a language or a model for describing this form of the sense. Effectiveness seems to vary with the person, the issue, and the language or model, so our process should benefit from a "the more, the merrier" approach to modalities.
Aside, I can't honestly say why the intuitive sense ought to be a magic ingredient. Here are two hand-wavy narratives I've heard along these lines. In IFS they call the process Self-leadership, so, the Self consoles the hurting parts within you; this to me evokes an evolved response as infants to turn to an adult for safety, providing resolution. And in Somatic Experiencing, there is the goal of preventing our human-cognitive process from disrupting our lower-cognitive processes represented by the felt sense, allowing their completion. What is a common aspect of theories of mind that they might both be getting at?
There are outliers, notably EMDR, which involves a kind of disrupted attention.
I also want to comment that the set and setting of the re-experiencing must offer us sufficient safety, at least partly because fear often elicits homeostatic responses. A posture of compassion seems to support this.
The conditions above are not a guarantee on their own. They just provide a fertile setting in which the seeker can eventually learn the true underlying process of self-change; they point in the right direction. In all cases, one should cultivate a practice, and keep groping for the thing to be learned until it clicks natively.