Internal structure of a 1 M_⊙ pre-CE RGB star with an H-free core of 0.271 M_⊙. The gray region corresponds to the envelope between the H-free core and the base of the convective zone, where the bifurcation point separates the future WD core from the ejected material. Dashed vertical lines indicate local criteria for selecting the bifurcation point: X_H = 0.1, maximum nuclear energy generation rate, maximum compression, and binding energy and entropy conditions (see Fig. 1). Red and black lines show the H profile and its cumulative value, M_H. The dark green line marks the innermost mass coordinate for which the residual M_H is sufficient to produce envelope expansion induced by nuclear burning beyond 1 R_⊙. The blue and cyan lines indicate the minimum M_H required for residual H burning to significantly impact cooling, and for the occurrence of unstable H flashes, respectively.