Drawing Tools can be applied, moved, and formatted, directly on a chart, providing tremendous flexibility. Many drawing tools allow you to analyze new data on a chart by extending the drawing object to the left or to the right of the chart. You can use them to call attention to any area of a chart you wish.
Trendline may be expanded in both directions, indicating the ratio of past/future prices.
If You extend a Trend line, its length is extended while the slope remains the same. If You want to extend a Trend line to future bars, you should extend it to the Right. If You want to extand a Trend line to historical bars, You should extend it to the Left.