That undercut and how much floor is visible really is amazing, and at the same time they manage to start the sidepod ramp very early, they then keep the centerline chubbier, for the pu to be able to breath,
you can se, by the sidepod design, just how much they want to direct the flow outwards towards the diffuser edges to help seal
edit,
seing the wet weather pics last year, and the flows im quessing something like this, and that the coanda helps pull the flow down towards the diff outer edges
or like this, which was more like last years flow, but then again, the flow was not aided so directly by the sidepod design like it is this year, so im not sure
Quess looking at the merc from last year and with similar sidepod style, most the flow connects with the floor flow and aids the diffuser, so more like the sec. picture