Because it’s a volume 3D image, you can rotate the image on those three different axes. Don’t worry about picturing which axis applies to what… just adjust them a little, and you’ll see how the baby’s face will tilt up-down, left-right, and clockwise-counterclockwise. This is helpful on the particularly difficult scans where the probe simply can’t get the best angle.
Do NOT overuse this. When you twist any of these too far, you’ll lose image resolution. Small adjustments won’t have much impact, but if you’re trying to turn the face 90-degrees up/down/left/right, you’ll get a really bad image. (This does not apply to the control that moves the baby’s face clockwise-counterclockwise, you can spin that baby to your heart’s content.)