Whereas a review uses a longer workflow that has multiple stages, most commonly the 3-stage or 4-stage, a check-in is designed to be quick and informal process. A check-in uses a shorter 2-stage workflow. This means the reviewer and reviewee can input their comments simultaneously. Your comments will still be hidden until you submit them.
If you need help on how to set-up a check-in with a member of your team, please click here.
If you need help on how to set-up a check-in with your manager, please click here.
If you're an admin and would like more information on this feature, please click here.