The Brush Keyframer

The Brush Keyframer is a utility to animate a brush (or animated brush) along a path.

You create the path by setting keyframes at important spots during the animation.

You can ,keyframe the scale, rotation, and opacity of the brush at any point in the animation as well as the position.

 
Before you begin with the Brush Keyframer, you should have an animation allocated.  Do this from the 'create' menu item under the animation menu.  Whatever image you have in your buffer will be duplicated on every frame when you create an animation, so it doesn't hurt to plan ahead to have an image ready.
Use the scrollbar at the bottom to scrub through your animation.  You can pick frames where you would like to add keyframes for your brush.  In the example above, there are 3 keyframes; one at the beginning, and at the end, and one in the middle.  You can add more if you want to define a more complex path.  At least two keyframes are required.
When you have picked a frame to add a keyframe, set your scale, rotation, and opacity they way you want them, and click the button to add a keyframe.  A red tick mark will appear on your scrollbar at the bottom, and you should see the changes to your path.  If you want to, you can reset the keyframe again, or if you want to remove a keyframe, you can press to remove a keyframe.  You have to have the scrollbar set on a frame with a keyframe (red mark) to delete a keyframe.

Press the button (clear) to remove all keyframes and start over.
  Align path causes a brush to always point in the direction that the path is leading.  This will override your rotation settings.   Animated brush offset is a value used with animated brushes.  It is the offset in frames to start drawing your animated brush.
  Motion blur creates an effect similar to the blur created when a fast moving object is photographed.  The shutter speed controls the amount of blur used.  A higher value will create a motion with a stronger blur.
If you have made changes to your brush and would like to see them in the Brush keyframer, you can re-load with the Get Brush button.
When you have finished adjusting your path, and would like to see the final results, hit the 'Render' button.  After a few moments, your animation will be built for you.  You can then exit or minimize the Brush keyframer, and scrub through your animation with the animation control panel.