  Recently, Jao et al. (2018) discovered a gap in the mid-M dwarfs main
sequence revealed by the analysis of Gaia data Release 2. They suggested the
feature is linked to the onset of full convection in M dwarfs. Following the
announcement of this discovery, MacDonald & Gizis (2018) proposed an
explanation based on standard stellar evolution models. In this paper we
re-examine the explanation suggested by MacDonald & Gizis (2018). We confirm
that nuclear burning and mixing process of $^3$He provide the best explanation
for the observed feature. We also find that a change in the energy transport
from convection to radiation does not induce structural changes that could be
visible. Regarding the very details of the process, however, we disagree with
MacDonald & Gizis (2018) and propose a different explanation.
