Think of a Store as a subset of some remote data source. In this case the 'remote' data source is actually local, but in could in principle be anywhere, accessed over HTTP.
On top of this subset is a delta of records added, records changed and records deleted. When we sync, we are applying that delta to our remote source via a Proxy. Sync is just the combination of the Create, Update and Delete actions. I'll think about the naming of that method though, suggestions welcome.
sync is good. I was just going over the current save method on a writable Store with a client, and mentioned that the 4.0 version would be the more logical name "sync" we all agreed that that was the correct term for the operation.