I have never used this option, but someone pointed it out to me today. It’s only available after one has clicked the first two points and one is in Line mode. The Help says this:
OK, but normally all one does is just type the next segment length and Enter to lock the length and click to set the direction.
What I noticed is using the Length option LOCKS the next segment direction parallel to the last-created segment. Which I guess could be useful, but why then is it named “Length”? Maybe I’m missing some essential function here?