Bitlocker in this scheme does not actually do any encryption from the system side (at least for data read/written). The bulk of Bitlocker in this mode is acting as a "Gatekeeper" since SED drives still need a means of access control to unlock the drive. When those are activated in this mode (with W8 and Bitlocker), the drive is initially locked and the system will only show a very small "shadow partition" under 200MB. This is where the W8 boot files are stored and the unlocking in Bitlocker happens with it interacting with TPM to pass a key to unlock the drive.