The Spin is a lot like the Chase effect. It can be thought of as several chases back to back. See the page for the Chase to get a feel for most of the parameters of the Spin.
There are two types of behaviors. Across Elements/Groups is the default behavior and is equivalent to how the effect acted prior to 3.6u1.
Each Element/Group is a new behavior that changes how the effect looks at the elements and targets them. Previously you could use the levels and spin across a group of Arches or similar props, but if you wanted to apply the same spin to each Arch in the group, you would need to put a Spin on each Arch specifically. Now you can choose the Each Element/Group behavior and the proper level to get an identical Spin on each arch. This also allows the possibility to Spin up/down a Pixel tree instead of just around it. Many combinations are possible using variations of the Behavior and Level options.