The theory underlying these indicators is that once a trend is in motion, it will continue in that direction. Technical analysis attempts to determine the strengh and the direction of the trend. The earlier that this can be accomplished, the earlier that it can be accomplished, the earlier the trend can be followed and of course, the more profit can be made.
Knowing the trend is equally as important as avoiding bad trade. Strategies such as buying the dips work well if the trend is going in your way. If is is not, the risk is higher and the potential profit is smaller.
Usually we use trendline to determine trend. When the price hit the trendline, we should see either it is bounce up or breakout happen. In the image below, you can clearly see how the price breakout from the trendline, then going down (from bull to bear). Here, you can make money.
Whereas, on the image below, you can see how the price bounce up from the trendline then bring further uptrend.