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/ Undo function is advisable to use provided the "New" copy is actually new, i.e. newly scanned, and reflects the current state of the Registry. Only in that case the proper return to the previous version of the key would take place. If time since last scan exceeds 10 minutes, you will see the warning. Of course, the time limit (10 minutes) is not a criterion reliable enough to ensure that the key is up-to-date, it might be as well you to control this yourself.
So, if you have scanned a new copy of the Registry and immediately ran comparing, without having been switched to any task before comparison, you may well admit that you have compared current state of the Registry to some previous state. Should it be necessary, you can return to that previous state correctly.
You may ask, why Undo is available when the "New" copy is not up-to-date? The answer is that it is not always necessary to change only fresh version, you may well need to change some old ones. For example, if you compare copy of the Registry scanned before installation of a program to the copy scanned after it, Compare function shows changes that appeared due to installation of that program. In that case Undo removes only changes caused by the program installation.
Contrariwise, Redo in this instance does the same changes to the Registry, as installation of the program did.
Unlike Undo, Redo is time senseless i.e. gives no warning if the time since New copy scan exceeds 10 minutes.
In general, it is advisable to use Redo only when settings of a program are damaged and you want to restore them and have the Registry copies scanned prior to installation of the program and after it.