If you have drawn a path and perhaps decide subsequently to
bend individual straight lines of this path, you have to
convert the lines into Bézier curves in order to do this. The same
thing applies the other way round as well, of course. Click first on
the relevant icon, then on the end of the line or Bézier
curve. When straight lines are converted to Bézier curves the new
curves will be rounded off immediately. In order to convert a path
made up only of straight lines into Bézier curves it is best to start
from the last point; then the curves will be rounded off in such a way
that no creases arise betwen sections.
This icon too remains selected after use so that you can convert several path sections during one work process.