One of my favorite buy signals on a trending pair is when price retraces enough on the short term to reach the second std deviation. (red line) Third std deviation even better.(orange line) For this to happen price must be moving quickly. A standard deviation is really just jargon for statistical likelihoods in a given timeframe. Of course counterintuitive to buy when price is falling so drastically but such is trading. The important thing to remember is the overall bias despite recent selling. I also like to buy after several consecutive bars in the opposite direction on the 4 hr chart. Sometimes when price reaches outlier deviations however it can also be a signal price will continue in that direction like a rocket. For this reason I would set a reasonable stop and just take a loss if price doesn't appear to be bouncing after a day or so.