Ratios of 38,2% and 61,8% are related to Fibonacci series and "golden ratio". Many processes naturally tend to follow the golden ratio. Human perception is somewhat skewed to consider it "natural". Here in terms of market changes, you could consider these levels as general-opinion "low risk" and "high risk" market reaction levels, just because there are people on the other side of the gambling table. Other people also look at charts and numbers, and other people as a group, tend to act according to golden ratio as well. Before 38% there's low chance of panic and change in trend, after 62% there's much higher chance. Of course, on the market any thing can happen, and it is NOT a rule to follow, it's just some estimation or guideline to keep in mind.