Correction : The angle step is 15 degrees, not 10 degrees.To rotate the oval or rectangle, drag the line that connects the center to the outermost oval or rectangle. If you hold down the shift key while adjusting the rotation angle, the angle is constrained to be a multiple of 10 degrees.
Note : 15 degrees is more convenient than 10 degrees, because the common angles are thus feasible: 30, 45, 60, 90 degrees etc.