A "W Bottom" as proposed by John Bollinger

• Price breaks to the downside and closes beyond the lower band in a classic breakout fashion;
  
• After making a bit of a bottom price pulls back a bit;
  
• The pair goes for another low, but this time it is not riding along the lower band. It makes another low but it's well within the Bollinger bands showing momentum has decreased to the downside
  

A buy signal "emitted" when the price goes above de resistance line (in green on this graph).