Every change you make to a design is saved as a new version of the design. This includes not only changes made to the design elements, but also any switches between 2D/3D, switching to a different navigation level, etc. All version saves are automatic.

To see all the version saves for a design, open that design. Click on the Branch icon (second button in the quick tools menu). Click on the Active Branch Tab. Here, you can see the list of version saves and when these were triggered, along with a comment about the nature of the event which triggered the version save.