In sketches, we know that the trim tool is pretty darn versatile. It can also do the functions of the extend tool (see:
What I was too dim to recognize was - just as the trim tool will 'extend' - or make a line longer- when you click and drag a sketch element, clicking and dragging a sketch element with the trim tool is also a really, really handy way to make the line shorter!
This is going to be really hard to describe because it is so visual in nature, but I'll give it a try:
Draw a vertical line (line 1) Draw a horizontal line (line 2) that crosses the vertical line. Make the left endpoint of line 1 significantly away from line 2. Make the right endpoint of line 1 just a tiny little distance away from line 2 (go ahead and add a dim if you need to - make it a few thousandths) This situation happens frequently in every day design - one line is just a little too long and needs to be trimmed back *(see footnote 1). What I've been doing for years, like a sucker, is zooming in really tight so I can select the section of line 1 that projects to the right of line 2 so I can click it with the trim tool to trim it back.
I just recently stumbled across the fact that I can click-hold the long section of line one (to the left of line 2), and drag that to line 2, and the tiny little spot to the right of line 2 will go away. Hey - an 'extend' that makes the line shorter, and I don't have to waste any time zooming in on anything! What's extra cool is that it (of course!) also works when you click drag to model edges and external sketch elements, and automatically adds a coincident relationship too.
All of this stuff is hard to describe with words. I recommend that you just take a few minutes, draw a whole mess of lines in a sketch, and see all the different ways you can click-drag a line to extend AND trim so you can get the full sense of what the tool can do. Be sure to see how it acts when you click drag past several other intersecting lines. Note: The SW help is not very clear or helpful on exploring all the permutations of this extremely useful tool.*footnote 1: lets forget about using a coincident relationship, which would be a fine solution in this simple sample. Lets pretend that the leg of line 2 to the right of line 1 is merged with something else in the sketch