If you're downloading Fallout 4 or Sea of Thieves on Xbox One, FastStart lets you play before the progress finishes.
During E3, Microsoft unveiled FastStart for Xbox One, a feature that optimizes downloads so you can start playing before the download finishes. A month later, the service has been made available with the latest Xbox One update.
The information comes directly from Microsoft (via Polygon) and applies to select Xbox One games. FastStart currently applies to all English-language games in the Xbox Game Pass, including Fallout 4, Sea of Thieves, Elder Scrolls Online, and more.
To be clear, FastStart won't make your download run any faster. It simply means the download is prioritized in such a way that you can start during the download, cutting your wait time by half on average.
The Xbox One update has many more features, including more search options, mixed share controller improvements, and full screen broadcasting on Mixer. But FastStart seems to be the one Microsoft hopes will resonate the most with players. Here's hoping it works as advertised.