Switch to Clip Viewer/Timeline viewer change the Movie track from its current view to the other one. The wording of the menu command may change, but the keyboard shortcut ( -E) remains the same.
Scroll to Playhead/Scroll to Selection are ideal for situations when you scrolled off to a different part of the movie to check something out ("Have I already used that hilarious clip of the baby's spaghetti face?"), and then want to return to the spot where you stopped playback ("Scroll to Playhead") or to the clip you had selected a moment ago ("Scroll to Selection"). iMovie returns to the designated spot without losing your Playhead position or clip selection. Keyboard shortcuts: Option- -P, Option- -S.
Zoom to Selection zooms the Timeline viewer precisely as much as necessary to fill the window width with whatever batch of clips you've selected. It even reveals just a tidbit of the preceding and following unselected clips, to help you gain your bearings. (Note that if you've highlighted only, say, a single 3-second clip, this command doesn't do much; it can't zoom beyond the limits of the Zoom slider.) Keyboard shortcut: Option- -Z.
Show Clip Volume Levels reveals the horizontal "rubber bands" that, in the Timeline Viewer, reveal the volume levels of your audio and video clips. (Page 228 has more detail.)
Apple mercifully turned this function into a menu commandbetter yet, a keystroke (Shift- -L)so that you no longer have to trudge off to the Preferences dialog box every time you want to hide or show these volume levels, as in previous versions.
Show Audio Waveforms makes those lie detector-style "sound waves" appear on your audio clips in the Timeline Viewer, so that you can identify louder and softer parts by eye. Details are on page 216. Keyboard shortcut: Shift- -W.
-E) remains the same. 