The functions of the Spectrum Window buttons are basically self-explanatory, but here are a few explicit notes. "Draw" draws or refreshes your spectrum/spectra in that window. "Expand" expands the bounded region so it fills up the whole Spectrum Window display. You would be wise to note that this is the preferred way to expand. As described below, you can also open a menu, then select View...Expand, but clicking to get the menu moves your cursor, thus altering your intended bounded region. This may just be a bug that will be fixed in a leter version, but for now it stands. "Previous" will get you back to the view you just left. So if you start with a full spectrum, zoom in on the left (amide stripe) half, then zoom on, say, the HN-HA region, then on a particular goup of peaks, clicking "Previous" backs you out one level to the HN-HA region. Clicking it again backs you out to the left half of the whole spectrum, etc. NVJ will remember a fair number of expansions, so you'll probably always be able to "back out" to a convenient level short of reverting to the full spectrum.
The "Center on Cursor" button will shift your expanded view so that it's centered on the location of the black crosshair. This button ignores the position of the red cursor and has no effect if you're viewing the full spectrum. The "Pan" buttons conveniently nudge the visible portion of your spectrum this way and that so you can see (or hide) peaks near the visible boundaries without zooming out and expanding on a new selection.