You will also see different behavior with the polyline tool if you click while the cursor is in motion. For instance if you click, move the cursor upwards, stop, click, you'll get a straight line.
However if you click, move the cursor up and right and click while in motion, the drawn line segment will begin to bend to follow your cursor. This will happen in the first mode without you needing to change modes, but its more for freeform drawing, for more precise control its best to change the modes manually as described above.