sacd-extract is used to extract DSD data from Super Audio CD ISO files. It is part of the sacd-ripper package which is used to extract DSD ISO files from certain unlocked Playstation 3 systems.
Typical usage might involve this command to extract all 2-channel DSD audio files from a Super Audio CD ISO:
user $ sacd-extract -2 -p -c -i /mnt/storage/media/audio/MyAlbum/MyAlbum.iso
Other usage examples can be seen in the Usage Examples section of the sacd-ripper README.
Sometimes, extraction will proceed quickly (tens of MB/sec), while for other albums, SACD extraction may proceed quite slowly (1-2MB/sec.) This is dependent on the endianness of the data that is stored on the Super Audio CD.