This mesquite cup is a variant on stacked segmentation to create wavy layes. It involves turning two green vessels where the inner one is taller and the two fit together perfectly on the sides. They are glued together and allowed to air dry. When they are finished turning after warping they fit together with a wavy pattern. The amount of warping is the main factor in determining the final thickness. Mesquite doesn't warp much so this one is very thin.
Notice that there is something that closely resembles a tenon on the base of the cup. If you select the large view, you can more clearly see that it is actually ...
... a tenon. Sneaky to get you to look at the large view.