It uses the fuselage as fuel, allowing for bigger payloads for a given amount of fuel and reducing space junk
https://phys.org/news/2024-01-rocket-uk-big-space-industry.html
Such a simple but brilliant idea! A UK company too. At the end of WWII and into the beginning of the cold war the UK had some amazing air and space development and they basically caved to the US and shut it all down.