It doesn’t move. It clips into the mudguard then you have to ‘adjust’ it to fit. On my BFE I can’t use it as the stub of a frame tube extends beyond the seat stays. If I fit it to my FS tho, I’ll cut it down a bit to fit as it’ll replace something I’ve already cable tied there to protect the many pivots on that bike!
I have an XL (mate bought two at the same time and went for biggest size)- it’s a bit long for hardtail but it’s astoundingly effective. Even compared to a V1 that was probably a bit short on a big 29er. After some truly grim rides this year, pack, riding kit (from the back anyway!) and seatpost/saddle all mud free.
Like I say a bit long but I’m not cutting it down till I’ve tried in other bikes. I don’t hang bikes up with mudguard on so it’s not really a problem.