Thanks OK for your deep review. I actually wrote the tip for Mac OS X, and forgot to mention that under Windows, the Ctrl key should be used to force the creation of a curved polyline, rather than the alt key. The fixed tip will appear in the next Beta.
Note also that the "control" points of the curve are not editable for simplification purpose, but their location are computed in such a way that if you draw a closed square, the resulting curve will be a circle. This tip should avoid to add an other tool able to draw circles and ellipses.