I asked Admissions last year: Laura assuaged my fears and confirmed that all applicants not selected for Regular Admissions automatically go to holistic review - no action required from your end.
That said, I have no idea how the holistic review unfolds and how much weight they place on grades. My personal theory is that the closer you are to the LSAT/GPA cutoff, the better your chances.
This is purely anecdotal (since I have no stats on all applicants), but they seem to be slightly forgiving when applying from McGill in terms of GPA. There are a few other students coming from B.Comm that got into Laval, Sherbrooke, UdeM with similar stats if you look at older threads.
Wish you the best of luck!