You can lay the model in a supine or prone position to perform a virtual cadaveric dissection or to run through a surgical procedure. This model position be done by activating Full 3D Rotation. To do this, you need to go to Settings > Preferences and activate Full 3D Rotation.
Once Full 3D Rotation is turned on, you can either twist with two fingers (touchscreens), or drag with ALT + left click to rotate the model around to a supine position. Guides will appear once the model is lying horizontal. You can then rotate the model to either supine or prone by swiping with one finger (touchscreens) or holding left-click and dragging.