The Separate tool is one of the SubD tools which you can use to separate the faces of an object.
Access and Use
The Separate tool is accessible in the SubD tools menu:
2. Pick the Separate tool (red circle with arrows)
3. Once you’ve selected the Separate tool, select the faces of the object you want to separate by pressing the front trigger on your drawing hand controller. If you want to select more than one face at the same time, press and hold the front trigger and hover over the faces to highlight them
4. Press the blue tick on your non-drawing hand to finalize the separation
Size of the selection circle
To change the size of the selection circle move the thumbstick on your drawing hand up (to increase it) or down (to decrease it).
Use History/Undo button to undo the selection.
Change to another SubD tool to cancel separate.