Hey cool trick,
never heard it called a flatty before though, in the UK thats a tabletop.
I think the selective colour works well here, it could be because it makes for a nice change from the heavily saturated and second shutter synch images that i'm used to seeing in most MTB publications this side of the pond. The sky looks almost gradient filled which is very cool. In a situation like this I would have zoomed into the subject further, but in this image I think the level of zoom also works well, giving a sense of the total scale of the jump.
Great effort, I look forward to seeing more MTB pics, as it's one of my favourite sports.