A Swap Value rule works a lot like a copy rule. Where a Copy Value rule takes data and copies it to another column, Swap Value is a two-way street. It exchanges values between columns. These are often used in conjunction with Set Value Rules.
For example, your company is doing business with another company called Zeta. There is a column called "buyers_account." There is also a column called "sellers_account." It's easy to set up a rule that says:
When transaction_direction =Buy
Then Set Value of buyers_account to 124XXX
Then Set Value of sellers_account to xxx999
Else Swap Value of buyers_account with sellers_account