If you look from above it seems that this wide slot is cut in from the front. The outboard part of the floor is connected to the relative simple bargeboards which have solid attachment upfront. Looks like a fairly stout and whole design package.
They ran the car before without it to get a baseline however it is quite clear that they intended to open the area up from the start. The whole bodywork is beautifully shaped and indicates that some air is induced from upflow above to manipulate the air streaming under the floor.
Said manipulation could help to 'seal' the floor or to reduce the drag of those wider rear tyres. Notice that Toro Rosso doesn't yet run slots ahead of said tyres like many other teams. Last year they too did. I suspect that they want to test this solution without additonal complications downflow to get another baseline to work from.
This might be one of the most important design paths of the 2017 season.