Dynamic is the pre-Wildfire spin algorithm. Mark the location of your mouse on the screen, spin the model around, then put your mouse back to the original location. The model is not in the original orientation.
Anchored is a new spin algorithm with Wildfire. You'll notice that as you spin the model around, there's an extra triangle joined by a line back to the first triangle. If you line these triangles back together, the model stays in the original orientation.