Only one of two things can happen when a price approaches support or resistance: the price can break through it, or it can bounce off and reverse direction. The same is of course true for trend lines.
1. Trading on a Pullback
If a chart is trending in a clear direction, and a trend line can be drawn connecting a series of relative highs or relative lows, trading opportunities exist when the price approaches the trend line. If the price bounces off the trend line and resumes the trend in the original direction, this can be an excellent opportunity to enter the market in the direction of the dominant trend. This is often referred to as buying on a pullback in an up trend or selling into strength in a downtrend.
Buying on a bounce off such a support line can be done through a limit order just above the support.
2. Trading a Break of the Trend
The second possible trade is the break of the trend line, which can be traded just as any other broken support or resistance line. If a candle closes through a trend line to the downside, as in the example below, the proper entry point would be to sell once the price moves below the low of the breakthrough candle.
This ensures that the short term force is in the direction of the break lower. The opposite would be true for a break above a resistance line.