The following options are available (from top to bottom):
Fast rotation
Rotations by 90, 180 and 270 degree are the most frequent ones.
Thus you are offered previews of the results of rotations by these
angels here. If one of them fits your need one click lasts to perform
the calculation.
Angle and "%"
With this you can exactly set the rotation angle. The "%"-setting
is equivalent to the decimals.
Preview
This switches on or off the preview in the right part of the
window.
Use current color
When rotating pictures by another angle than 0, 90, 180, 270, 360,
new and empty areas are created. If this option is activated, the
current color is used to fill these areas, black otherwise.