We use Google Drive to create and save a file on your account so you will be able to open the same checklist on all your connected devices.
And guess what?! You can share it! Well, yeah is not a big deal, but it come for free, yes like everything else, what I mean is that we didn't had to do anything for that, right? So basically you connect with Google, we create the file, you go to your Google Drive folder, try to find the new file, I have no idea where it is or what the name will be, the guy who did this didn't tell us, once you've found the file share it with your friends. Done! When your friends connect their Google account they should see the same checklist status. Magic isn't it? No! That's technology fool!
Also, we will not use any of your data, we didn't have any kind of database or save something somewhere in any way, the file we create is yours and yours only and is the only thing we are able to do right now.
Oh, and just want to remind you that if you still not believe it, everything is open source and you can check the github repo here (feel free to fork it and do awesome things, or help us make it better).