It works like this:
1. You link your Twitter feed to your Amazon account in your Social Settings page.
2. While scanning the Matrix that is Twitter, you see a Tweet that contains a link to a product on Amazon (Question: Who is Tweeting out links to products on Amazon? Is this only an advertiser thing or is it something that actual humans do? “I’m feeling #great today #yoloblahblahblah… and buy me this thing on Amazon”)
3. Reply to that Tweet with the hashtag #AmazonCart and it gets added to your shopping cart so you can buy it the next time you go to Amazon, which is presumably when you’re coming down between Tweeting jags.
So if your Twittering is so urgent and absorbing that you can’t be bothered to take the time to click on a link and follow it to Amazon right then and there, this may be a mildly easy way for you to delay the inevitability of having to leave Twitter.
Here’s a video that explains it all, or maybe details the heretofore untold secrets of the universe… We didn’t make it all the way to the end so we can’t say for sure: