I think what Ben means is that you don't want to dump a lot of acid in all at once to lower the pH by a lot. It is too easy to overshoot your target! Also it really takes about 24 hours for the pH to stabilze after ajusting it. It is much safe to make a small change and keep doing this over a few days until you get it where you want it and then add just enough acid to keep it there for maintenance.