By default, the rear command dial rotates to zoom in and out of the selected image during playback. Zooming all the way out goes from full image, to images taken that day, to all images. The front command dial below the shutter button goes back and forth from picture to picture, you can scroll through all the pics on the card.
Using a manual lens while shooting, the rear command dial, when pushed, zooms in and turning it gives you smaller or larger zoomed in view. Then the front command dial does third steps shutter speed, whereas the rear dial does this when not being used for zooming in on manual focus. Seems a little confusing, there is probably a way to change it, but I don't know how.
Hope this helps.