Wow that is nice! Where can I find more info on free form piercing , I have seen it before on here and it is always impressive to me. Good work!
The closest thing I've seen to any instruction on free form piercing is in the DVD "Surface Design on Thin Wall Turning" by Binh Pho, but the free form part is just part of one chapter.
http://www.woodturnerscatalog.com/s..._on_Thin_Wall_Turning___pho_surface_vid?Args=
I'll describe my design method. I'll skip an explination on how to use the tool itself (NSK Presto, Turbocarver)
Before trying this with a tool, do the following.
1) Print some representative line art from Google or other source.
2) Draw a large circle around the image or arc across the top representing the rim of the turning
3) With a pencil, following the outline of the image draw a series of similar sized, but varying shape blobs where one edge is the image, and one edge is the adjacent blob. Maintain an inter blob rib of 1/16" to 1/8". Draw each blob in a clockwise direction (You need to use the tool this way and this gets you in that habit)
4) Repeat step 3 for the rim
5) With a pencil, draw another line of blobs offset from either the image or rim blobs, once again maintaining the 1/16" to 1/8" ribs between the blobs.
6) Repeat 5 until you have thoroughly "blobed" the region between the image and the rim.
What you will discover is that you need to put some thought into blob shape when close to sharp corners (like the tips of the leaves, or the gullies between tips in my bowl).
You will discover that similar thought is required when a line of blobs is about to intersect another line of blobs. The end of step 3 when drawing the last couple blobs to make them look right next to the first blob is one example. Another example is when the rim blobs are about to intersect the image blobs.
This free form piercing does not have to be between and image and a rim. The blobs can form an image like I did here
http://www.penturners.org/forum/showthread.php?t=69922
In this case the image was a simple spiral, but the image could be more complex like this leaf
http://www.penturners.org/forum/showthread.php?t=66829
Where the freeform piercing is inside the maple leaf rather than outside.
All three of these used the same fundamental method of blob design.
Once you a comfortable with drawing the blobs on paper, you just use the same basic design technique with one of the super high speed (300,000 RPM+) rotary tools. These tools are the rough shape and weight of a pen so the techniques you develop when drawing blobs will feel similar to cutting blobs with these tools.