from what i have read, and what TeaVicar? has said, i understand that ET is a form of maternal/paternal transference. The best explanation and information i have come across about transference and countertransference is from Dr Sebern Fisher, specifically in regard to developmental trauma and from more of a neurophysiological perspective. she talks about it in her book: Neurofeedback in the Treatment of Developmental Trauma | Sebern Fisher

here are a couple of excerpts from her book about transference you may find helpful:

Transferrence in developmental truama begins in the felt experince of 'no mother'. Motherlessness is the lowest ring of hell for a baby or small child, creating a background fear of death of obliteration.

Transference expresses the unmet need of the infant. It is about dysregulation. In this paradigm we can look at transference as a nervous system seeking its own regulation from the other, almost always on its own dysregulated but insistent terms. Transference relationships that arise with patients who have suffered developmental trauma are fraught with expectations of mothering and intensified by affect dysregualtion, distrust, and unbearable hunger. For both the patient and the therapist this transference can be challenging to manage.