btfs can be used as /init for constructing a run-time environment before running the real init.

It supports extracting compressed (.cgz, *.cbz2) and uncompressed (.co) cpios located on a compile-defined directory (/).

Also, it creates a tmpfs filesystem as the new root, if / isn't one already and the purpose of the initramfs isn't for starting a system in another location (disk, network, etc.). That will allow for the data on / to be swapped to disk when swap devices are activated later.