When this option is not empty a swap file is synced to disk after writing to it. This takes some time, especially on busy unix systems. When this option is empty parts of the swap file may be in memory and not written to disk. When the system crashes you may lose more work. On Unix the system does a sync now and then without Vim asking for it, so the disadvantage of setting this option off is small. On some systems the swap file will not be written at all. For a unix system setting it to "sync" will use the sync() call instead of the default fsync(), which may work better on some systems. The 'fsync' option is used for the actual file.