Hi Gavin,
Welcome to the Forum.
When you rip a CD using the Brennan B2, the B2 ALWAYS rips the music off the CD as a WAV file.
You can set/choose what format you want these WAV files to end up as using this front menu function.
Then
If you leave the B2 powered on and idle (playing no music or open Web UI) it will begin to automatically compress any WAV files it finds to the Format you have chosen (by default FLAC - the "best" option).
If you use the B2 at any point while this is happening, no worries, the B2 pauses the compression and does what you ask. It will start compressing again when it is next idle.
If you want you can also kick start compression using this front menu Function.
Compression can take a long time
Fred