A resync, or full resynchronization, is a one-time process activated manually by the user to bring Structure and some other aspect of issues to the same page. Resync typically scans all issues that may be affected - contrary to the incremental synchronization, which inspects only issues that have been changed.
For example, running resync on a Saved Filter synchronizer (in Add mode) runs the related Saved Filter and makes sure all issues from the result set are in the structure. When the same synchronizer is working in the background, it checks only those issues that have been changed.
Resync is also different from incremental synchronization in that it has a direction. The incremental sync tries to apply changes on both sides to the other side, if possible, depending on where the change has happened: with JIRA Agile (GreenHopper) synchronizer, if you change the rank (issue position in backlog on the Planning Board), its position in the structure is also changed; and if you change the position in the structure, GreenHopper's rank is changed. However, when applying Resync, you need to choose which side of the data is to be taken as the final version and which is to be updated.
Resync can be run:
A synchronizer may support resyncing in only one direction. For example, Saved Filter synchronizer, which adds issues from a saved filter result, can only sync into Structure.
To run a resync:
Resyncing in a wrong direction may lead to data loss! Please make sure you understand that you're doing the correct thing and confirm running the resync when a confirmation dialog appears.
If the synchronizer is currently running an incremental synchronization, the resync will wait until it finishes.