I just position my hand above the space bar and the moment I see any movement, I try and press as fast as possible

Not very good advice, but it managed to get me a score!

The other thing you could do is just randomly hit the space bar -- it might take a long time, but eventually you'll be lucky and that will be the moment when the Darblat slides by... I think this must be what the trophy winners do, unless they have some way to slow down the game o.o