From historical data, the probability of a move greater than 5% to the downside after a confirmed Hindenburg Omen was 77% [The Wall Street Journal 8/23/2010 article cited below states that accuracy is 25%, looking at period from 1985], and usually takes place within the next forty days.
Because of the specific and seemingly random nature of the Hindenburg Omen criteria, the phenomenon may be simply a case of overfitting.