Took me a while to figure out what was going on. The description wasn't clear that shift/LMB switches the direction of gravity, so I couldn't form any intuition of the block's behavior for the first 20 attempts or so. After that, there are a few spots where you seemingly can't avoid falling off the screen, thus requiring careful planning planning of the route, which (to me) somewhat defeats the purpose of making this a speed running game. There is no forgiveness for getting your route wrong, and no visual cues to let you know you should be approaching high or low. Maybe I'm just missing something.
The mechanic is really cool once you figure it out! I just wish it was more intuitive.
EDIT: So I realized there is a constant unintended Left-move input, making it harder than it should be. At the time I thought that was part of the gameplay, but I've since seen it in a few other games. It might be a Win10 thing, but it's a bummer because I want to finish the level!