This function makes it easier to combine curves and straight lines. Furthermore, this drawing mode might be more intuitive for people new to OCAD.
First activate the option Curve mode: Change to straight line mode when clicking in drawing area in the Preferences -> Drawing and Editing tab.
Start drawing the curve as usual. To draw a straight line, just click in the drawing area. This way, you can easy switch between straight line mode and curve mode. Double click the left mouse button to finish drawing.