Representative evolution of two 5 M_⊙ donor stars, one that will experience a DDI, and one that will not. The donor stars fill their Roche lobes at the same moment in their detached evolution (at a size of 15 R_⊙), but lose mass in different binary configurations. The DDI system, shown in red, contains an initially 1 M_⊙ companion star, whilst the stable mass transfer system, shown in blue, starts with an initially 1.5 M_⊙ companion. Points of interest are labelled with letters, which are explained in the text and Table 1. In the lower left panel, the dotted line shows the mass transfer rate based on the Kelvin-Helmholtz thermal timescale at point A.