trims the 3D graphics frame while the graphic is being rotated.
Details
By default the 3D graphic frame increases in width and height to accommodate the graphic as you rotate it, and trims the frame once you release the mouse button.
To trim the frame as you rotate the 3D graphic, right-click on it and select Trim Bounding Box.
Trim Bounding Box can be found in the contextual menu for 3D graphics.