The xDai bridge is used to move Sai/Dai to the xDai chain, and xDai back to the Ethereum mainnet.
Using the xDai Bridge
Instructions for usage with the UI are available on the xdaichain.com site
The bridge can also be used without the UI. Instructions are here.
It is possible to move Sai/Dai to xDai and back to an alternative receiver by calling the bridge methods directly how it is presented in this manual.