If the gradient is set as being to the East for example, then whatever is drawn next will have the current color in the SouthWest corner and the second color in the NorthEast with a smooth transition in between. This shading will work with anything that IconPainter can draw including curves, text, shapes, lines and so on. Flood-filling is also subject to the current gradient setting.
A good example of this shading is the EquitySoft logo at the bottom of this page which has a transition to the NorthEast for the circle with a transition in the opposite direction for the text. This kind of transition would be almost impossible to do successfully by hand and the results can give an otherwise dull icon an exciting, 3-dimensional feel.
The fast method works out the size of a box that fits around the shape or text drawn then uses this box's dimensions to do the shading. This is very fast and is recommended on large images however it can be a little inaccurate for some shapes and for some fill directions but can still produce the desired effect.