The plate is held and thrown vertically, and spins in the vertical plane. Jugglers will recognise this as the way that rings are usually thrown.
From the TricksWithHats archive
Hold the plate by it's edge and spin it so that the rim stays in contact with your thumb. After a single spin catch it again. Once you've mastered singles try longer spins. This trick is very versatile and can be used to add lots of flourishes.