An upward trendline, by definition, develops as a stock steadily appreciates over an extended period of time.  During that period the stock vacillates between the lower and upper boundaries of trendlines which can be drawn connecting a series of recent highs or lows.  In order to ensure the overall health of the stock, the lower boundary should not be violated.  Technically, if the lower boundary is violated this signals that the trend is deteriorating and bears are gaining control, making the odds start to favor the possibility of further downside testing. 

MSC Industrial Direct Co Inc. (MSM -0.93 or -2.03% to $44.93) gapped lower today and closed in the middle of its range for a loss on above average volume. This issue was recently featured at $40.66 on January 3rd, 2006 in the CANSLIM.net Mid-Day Breakouts Report (read here) as it was emerging from a relatively flat base. Since then this issue rallied over +10% and traced out an upward sloping trendline.  After an impressive run it is normal, actually healthy, for stocks to pull back in an orderly fashion.  Ideally one would like to see volume ease as this issue finds support near the lower boundary of its upward trendline.