Yes, you see head and shoulders patterns from time to time on individual issues but never for the entire market! This is incredible and is a serious signal for more declines to come. The chart speaks for itself: Also note the "death cross" -- 40 day MA crossing down under the 200 day MA. A lot of times you'll get a relief rally after the neckline is broken back up to the neckline, to be followed by another decline. We'll see. This is not good, folks.
Habit. A lot of people use the 40, a lot use the 50. I don't think it makes much difference. The 40 will cross the 200 a little faster, though.