The sync command forces any information on its way to the hard disk to be written out immediately. This is useful if the operating system has crashed, or has been interrupted without preserving all data first.
sync actually returns control to the operating system, which then performs the data saving operations. After the disk data has been synchronized, the operating system begins to save a core image of itself. If you do not need this core dump, you can interrupt the operation with the Stop-A key sequence.