I think the point he's making is that if you clamp something to the table and move the table in only xyz, you can't undercut the shape, which is kind of obvious really. Clearly you could use a ball nose cutter and make a shape which is everywhere convex or concave wrt to x and y, but it couldn't be concave wrt to z if you see what I mean. You could imagine a tool which is a bit like a tee slot cutter, but even them there would be limitations on undercut. On the other hand if you could rotate the work around the x and y axes then undercut is obviously possible.
Sent from my iPad using Tapatalk