Gidgit, In the video Pixolator was using a mask to extract sub-tools.
Draw a 3D plane onto the canvas and enter edit mode.
On the tools palette convert the 3D plane into a polymesh for sculpting, subdivide it a few times if you want a detailed mask.
Holding Ctrl will bring up the masking brush, there are several to choose from in the brushes palette.
Begin painting a mask onto the polymesh, once you have your shape defined open the sub-tools palette and press the extract button at the bottom of the palette.
A new sub-tool will be created in the sub-tool palette. You can manipulate it with the transpose tools. Clear the mask from the sub-tool before you transpose or it will deform,
Short cut keys Ctrl+shift+A to clear any masking from the selected sub-tool.

You can make as many parts as you want from a single 3D plane, just clear the mask and your ready to go again.
In the video Pixolator was masking around the outside of the shape he wanted to create, then he would invert the mask before the extraction.