Half-swiping is a feature of the Snapchat app that was actually quite simple to use, even though it wasn't part of the original design. Users on Snapchat know that you swipe left to open a message, but they also discovered quite quickly that if you half-swipe, you can see the message without sending the other user a notification that it's been seen. It's an effective way of previewing a message before deciding whether you want to respond.