Part of the Power Reports feature is the ability to select a custom comparison period - this will allow you to specify your own period where you'd like the metrics to be compared. By default, the comparison period is the preceding time period of the same length.
Changing this default period will allow you to specify your own comparison period which the metric's evolution will be based on.
You can always go back to the default comparison period by clicking on Back to default comparison period.
Finally, few things to keep in mind:
custom comparison period can be max 1 year long (same as for the reporting period)
you are not able to select a comparison period earlier than your data retention period
if you change the main report period in your view, the comparison period resets to default