As I understand it Popcorn Time torrents and seeds a file sequentially, allowing you to watch the video as it downloads and removes the video from your device when it's over. It's more similar to torrenting a file yourself, but doesn't store the file when it's over. In contrast this torrents the file to the server and allows you to stream the content over your browser, so it isn't your device doing the leeching and seeding. You just watch it all happen.