/X
Perform free-space consolidation. Free-space consolidation is useful if you need to shrink a volume, and it can reduce fragmentation of future files.
/T
Track an operation already in progress on the specified volume.
/H
Run the operation at normal priority instead of the default low priority. Specify this option if a computer is not otherwise in use.
/M
Defragment multiple volumes simultaneously, in parallel. This is primarily useful for computers that can access multiple disks simultaneously, such as those using SCSI- or SATA-based disks rather than disks with an IDE interface.
via
Defrag from the Command-Line for More Complete Control
