Marcelo finds the finish regardless of which leg his opponent captures: The important thing to remember when pulling an opponent down to the mat with their head caught in a Guillotine Choke is to not allow them to pass towards the side of their defense; i.e., always make sure their head is underneath your body and not above (like when trying to finish a Loop Choke). Place whatever guard variation you can manage to prevent them from escaping out towards the side that will let them bring their head to the surface. This can come by way of any number of Half-Guard variants, e.g. 3/4, High-Low, Empty, Reverse-X, et al., depending on which leg remains free after the pull.