Ahh! You going to make me have to draw a picture.
I'd understand better if the blue line was extended as a dotted line to the back of the dotted green area. It looks like it would hit the front axle line, not behind the rear wheels.
So I guess the proposal is an L-shape not a V-shape. I suspect it's too small to matter. Here are some examples, a Ferrari that shunts air sideways into an external duct
A race car that does what you want your idea to do.
The diverter expands the air, then it is sliced off and expelled behind the front wheels.