The reasoning behind it is, to stay away from the market when volatility is either to high or to low. I looked at the average range of normal conditions and I came up with these numbers, maybe it needs adjustments, but the basic idea is in place. If you get very fast moving candles you'll be better staying out, because it might all fly back as in news times. On the other hand if its to low you can also run in a problem with stagnation which can move back and forth going no where which usually ends in bad trades. Hope this helps, good luck.