When selected, the waveform display is calculated based on the sample-by-sample peak level. Peak view is traditionally how Pro Tools calculates the waveform overview and can be used for normal or rectified views. Peak view clearly displays any clipping in the waveform.
When selected, the wave form display is calculated according to the Root Mean Square (RMS). Power view can be used for normal or rectified views. Power view is useful for better seeing the characteristics of the audio in the waveform representation when zoomed out beyond the sample level. For mastering applications in particular, it can be more revealing of the sonic characteristics of the audio than Peak view.