Reaction time.no seriously. Honestly, I don't even play nomod sometimes. I just put on hardrock, sightread it a few times and I get it down. Actually, I've gotten some easy hard rock scores on the first try by just sightreading random stuff like this.As for stacks, I've realized that I'm actually decent at reading hidden, and much of the time I don't even need the approach circle to read stacks and back and forth notes. Like, it's become intuition sometimes as to where an "unseeable" note is. Also, I can read the automatic osu! stacking pretty well too.So for HardRock, basically it's just reaction time, as it is the same with AR 10.3 for me as well if I ever do play those types of maps. For stacks, I have no real way of doing this, but being able to read the automatic osu! stacking and being able to read weird back and forth patterns without the approach circles might help. Only I see of doing this practicing hd+hr or just hd on maps that have these weird patterns, which I don't have any in mind right now.Edit: if I recall correctly this map might have some hard stacks to read on hidden (but they aren't like direct stacks, I mean like stacks that are under sliders and such). http://osu.ppy.sh/s/52547# but when I say I'm decent at reading hidden, I mean like I can sight read this on hidden on the first try and get like 95% accuracy still.though I'm not at home right now so I can't really actually play the map and test whether this was the right map i was thinking about.