These alerts appear whenever a stock crosses important support and resistance lines.  When a stock trades a lot near a particular price level, but never goes above that price, we draw a resistance line at that price.  When a stock trades a lot near a price level, but never goes below that price, we draw a support line at that price.

Support and resistance lines are not an exact science.  Most interesting stocks always fluctuate by at least a few cents in even the smallest timeframes.  We use proprietary filtering algorithms to determine the best place to draw these lines, and a few prints are always on the wrong side of the line.  We use related algorithms to determine when the lines have been crossed.  Again, a single print can cross the line without causing an alert.  Sufficient volume is required to cause the alert.

More options related to these alerts are listed below.