FFU firmware contains pre-installed Windows 10 on ARM systems and promises to "sit" in Windows 10 Out-Of-Box after the flash. The firmware that is released is ready to start and requires no further action on your part to get Windows 10 running on your device.
These images do not contain Window 10 Mobile, and flash will delete Win 10 Mobile completely from your device. So it would be best to keep backing up your data for each case.
With FFU firmware, you can install Window 10 using the 32GB eMMC space.
Take some time to read the flash instructions before passing the FFU firmware to your device, as a special step is required to ensure that Windows 10 is working properly on your device.
The project has recently released images of Window 10 on ARM for the Lumia 950 XL. LumiaWOA has the firmware you need and you can download it to install using WPinternals. Installing Windows 10 Mobile will be replaced.
So if you have a Lumia 950 XL you can try hack.