Review mode just gives you an organized way to look at some or all of your projects on a regular basis. The main benefit the feature provides (as opposed to the actual act of reviewing the project) is keeping track of when you last reviewed each project. If you have a project you want to review and mark as reviewed, but it isn't yet due to be reviewed, you can view it in the Project view, tap on the project header, and select Review from the popup. That will switch you to the Review mode with that project selected, even if it is not yet otherwise due for reviewing. There is no way that I'm aware of to conveniently do this to a large number of projects; you should use the Contact Omni command under the gear menu to submit a feature request for that.
As I said, the Review mode just gives you an organized way to look at your projects. If you want to do a sweeping review even though it isn't time yet, you can do it with only a few extra taps. Tap on the > button in the sidebar next to Projects to get a list of projects in the sidebar. Now tap on the top project in the list, and look it over just as you would in Review mode. When you've done so, tap the next project in the list. If you come to a folder, tap the > to go into that folder, and when you've reviewed everything in the folder, tap the <Projects| button to go back up. Repeat until you've reviewed all of your projects. At that point, if there are any projects which haven't actually been marked as reviewed, you could go to Review mode and quickly mark them reviewed. Review mode just makes it easier to do this process with interruptions (and not having to navigate the folder hierarchy manually, if you have one).