You can select back-up to be immediate, so any time you save a new or updated file to one of your designated folders it will also be copied to the online store. Alternatively, you can choose to backup files at regular intervals, say once a day.
Backed up files are held on Amazon's servers. While we have little doubt these are secure, given the volumes of retail traffic they have to handle, we would be happier if data wasn't held by a third-party and wonder who would be liable, if it did get hacked.
If you choose active backup, the process goes on in the background while you continue to work. There is a small resource hit, but since work files tend to be small and only saved occasionally, it isn't too noticeable in normal use. You can specify to hold two or more versions of files, giving you an archive for reference.
As well as sync and backup, two other options are available in BeInSync: Share and Access. Share provides a way of sharing specific folders not just with others on your network, but via an Internet link, in effect making your PC a Web server for a closed user group. Everything is encrypted and people you give access to just see a list of files, a bit like looking at an FTP page.
Access is the online storage trick which enables you to get at your backed up files from any machine with Internet access or to go through the site to your own BeInSynced PC. The BeInSync site is secure so once you're in you can look at and copy your files to a remote PC, saving the hassle of having to carry around a PCAnywhere-style application in order to get at your stuff.
If you want to try BeInSync, a free version of the program is available which can sync five folders between two machines and provides 1GB of online storage. If you're needs are modest, you may not need to pay anything for BeInSync.
As an online back-up resource, BeInSync has an obvious restriction, in comparison with Carbonite. The Carbonite backup size is unlimited, while BeInSync has an initial limit of 5GB and you have to pay for more. In its favour, though, BeInSync does the synchronisation thing so you effectively have several backups between your own PCs. Everything you select is held on up to five machines and kept in sync automatically each time you click Save. This is a clever product but, at £30-odd ($60) a year, you need to be sure a simpler local backup solution won't do the job just as well.