When you send any amount of STEEM / SBD using the swap functionality, @tipU will perform exchange operation on the steem internal market and send you back the other currency. Thanks to this you can quickly exchange SBD to STEEM and STEEM to SBD at the market prices.
You can also send STEEM / SBD to @tipU with memo: swap for the same effect.
Fees:Currently there are no fees for the operation, it's free :)