Needlecasting is a way of transmitting, much like radio waves and probably quantum in nature, data to other and far distant locations wirelessly. Needlecasting somehow manages to transmit data at FAR faster than light speeds, another Martian technology humans have learned to use but cannot explain why or how it works. For example, data can be transmitted from Old Earth to Harlan's World many many light years away virtually instantaneously. That would include any data: the kind we transmit around on earth today and, though not primarily, DHF data.
Points of interest
Double-sleeving is very much illegal, and whilst it is perfectly legal to have copies of yourself stored remotely, it is permissible to do so when you only deploy your consciousness in one sleeve at a time.