Accessed from the Snapshots button, or Tools Create Registry Snapshot.
A record of the current state of the registry will be made - actually in the form of an InterVations database. Any existing snapshot will be overwritten, so it is only possible to have one snapshot in existence at any time.
The state of the registry at a later time can then be compared with the snapshot to show what changes have occurred.
NOTE - the registry is a dynamic, shared resource, whose contents may be changed at any time by software. So there may be changes during the taking of the snapshot over which we have no control. To reduce this problem, whilst taking snapshots (and compare results), have as little software running as possible.