Remotely accessing files on one Mac from another has been possible for years, but now a Mac version of the iTwin (US$99) device is intended to make it easier to securely send files back and forth over the Internet. After giving the device a try, I found it to be much more confusing than just using something like Dropbox for file sharing, as well as being poorly implemented on the Mac. Read on for a description of how it works, along with my take on why the iTwin might not be the sharing solution you want to use.
Using iTwin for Mac
The iTwin for Mac looks like a double-ended USB flash drive that pulls apart in the middle. It doesn't actually have any onboard storage; instead, it uses the Mac (or PC -- it's cross-platform) on the other side of an Internet connection as storage. Have a 2 TB drive sitting in that iMac at home with plenty of free space, and want to use it as storage for your MacBook Air while you're on vacation? No problem, as long as you have an Internet connection available.
To install the iTwin's software, you simply plug it into an open USB port. A DVD image appears on your Desktop, you open it and click "LaunchiTwinMac" to install. This is a bit more than the "automatic install" that iTwin promises, and the installation leaves a folder in your Applications folder filled with a bunch of detritus (see below) one would normally associate with a Windows application. There's also a iTwin preference pane installed in System Preferences; basically, it allows you to uninstall the iTwin software from your Mac.
There were no more instructions about what to do, either in the installation notes or on the iTwin website. I simply unplugged the iTwin (it never squawked at me about not dismounting it first), and then plugged it back in. The second time, I was asked to give my iTwin a "friendly name" to identify it and also to provide an email address for a disable code. This disable code allows you to disable one half of the iTwin pair should you accidentally lose the other, thus keeping unfriendly eyes away from your digital stuff. At the same time, unique encryption keys are created for both halves of the device, ensuring that the only way you can get access to your Macs is by having your hands on both USB dongles.
At this point, an empty folder called "Steve's iTwins Local Files" appeared on my desktop and a tiny green icon appeared in my menu bar. I figured it was about time to pull off one half of the iTwin pair and plug it into my MacBook Air.
After going through the iTwin software installation process on the MacBook Air, I was greeted with a second icon on my desktop, this one called "Steve's iTwins Remote Files". I decided that I would use that to transfer a photo to my iMac, so I dropped a file on it. Instead of putting the file into a specific folder on my iMac, it put it onto the desktop and also put a copy into the Local Files folder. That got a big "huh?" (actually more of a "WTF?") from me.