Using Masks and Layers—Painting, Filling, and Color Tools - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 249

To avoid setting unneeded control points, take a closer look at the figure
you would like to extract. Where are the corner points? Where are the so-called
inflection points of the curves (points where the curve changes the direction
of its curvature)? In most cases it is sufficient to set control points at the critical
points of a contour. Now it is possible to draw spline lines, bend curves from
those control points, which can be applied to the curvature of the contour.
Furthermore, you can insert additional control points as needed.
As you can see in the example, control points were placed only on spots
where there is a bend in the figure or where the outline changes its direction.
Even with curves at regular intervals you will need only a few control points.
For example, with a circle, you only need three and the outline is developed
from the control points.

3.11.3 The Path Editing Mode

Continue using the Paths tool in the Edit mode. To
position the anchor points, Polygonal must be selected
in the tool settings.
You may want to take advantage of the Zoom tool
and zoom into your image. Feel free to add more path
control points by clicking the mouse on any path line
while holding down the Ctrl key. You can also delete any
superfluous control points by clicking on them while
pressing Ctrl + Shift .
When you are satisfied with the positioning of the
control points, convert the path segments to curves so
that the path line will precisely trace the contours of the
object. Stay in Edit mode, but click the Polygonal setting
to disable it.
If you click on a control point and drag while
holding the left mouse button down, two empty
squares at either side of the point will appear: These are
“handles” that can be used to define the direction and
bending of the path's curve. Each control point has two
handles. To adjust a curve on a specific section of the
path, you simply move the handle of a nearby control
point with the left mouse button. Play around with the
handles until you've succeeded to create a path that
exactly traces the contours of the wine glass.

Figure 3.65
Path control points and linear segments on an object.
Handles are drawn out from a control point to bend curve
segments (highlighted red).