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Load
an existing image, something with some form of shapes or distinct
patterns you can identify. Even something as randomly created in PD
Pro's filters such as
Filter> Render> Cold
Lava
I recommend using Image> Store
Image... to stash away a copy for re-use.
Having select a particle brush, such as the preset named Garland, click
on the "FF" button to use the loaded image as the basis for a force
field.
After a few moments you'll see the interface panel:

Scroll through the text box at the top of the panel, and read it. This
will serve as a quick reminder of what it's about.
Now, if you click "Free", the force field will be released and
disabled. You can then close the panel. You can click "FF" again to
return to the same forcefield (it will simply re-evaluate it, and if
it's the same image it'll be the same forcefield)
Draw a stroke without the force field enabled.
Here's an example of a quick scribbly line from left to right, jut one
brush stroke!
^ click image to enlarge
This is a normal Garland appearance from the particle brush.
Now, click the stored image of the cold lava or whatever you want to use as a force field. Or load
the image again. Then, either paint a new brush stroke, or better yet, use Shift-A to again render the most
recent (prior) brush stroke. Or use the Stroke player from the
Animation menu to the same effect.
^ click image to enlarge
By using the same brush stroke you'll get a better idea of what happens
with the presence of a force field, as you can compare side by side the
before FF and after FF. Click the above images and compare.
Evidently, there's already some wilderness happening with these
particle paths, they appear less tamed, more erratic. The force field
is intially set to rather low level.
If the presence of the image (cold lava)
that was used for making the forcefield is distracting, you can also clear the image and hit Shift-A again.
^ click image to enlarge
More Power
Now change the value on the first slider. The particles will be yanked
around more forcefully. It's like they have a higher startup (initial)
velocity. This is the strength at which the forcefield affects the
particle's velocity.

^ click image to enlarge
More (conducive) Drag
The other slider forces the particles closer to the guiding lines of
the forcefield, i.e. along the flux path. If the Power slider is
reduced, and the drag is increased, you'll see the particles sort of
tamely contained in their origination spot, not venturing very far away
from where they started.

^ click image to enlarge
More Force and More Drag!
And now it gets really interesting, when you use both sliders for more
force and more drag. Soon enough you'll start recognizing the shapes of
the forcefield, i.e. the image that was used to define it.

^ click image to enlarge
In the above example, we only used the first mode - Contour - to tell
the system what aspect of the image to use for forcefield calculation
and application. Instead of the shapes (which is what contour lines are
correlated to), you could make it follow the Hue, or the Value (such as
brightness).
We'll see a few more examples later. But let's see an example of what
you can now easily achieve, with just this one brush stroke,
re-rendered a few times, with various parameter changes, including
switching to other brush metrics that randomly place flowers over
several layers of moss.

That's it for a first intro, let's explore some
more ideas soon.
Here are a few more basics reminders for inspiration:
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