Thank you to www.secretsanta.com for this definition of Yankee Swap:
In Yankee Swap each participant brings a wrapped, unmarked gift and places it in a designated area. Guests are given numbers as they arrive, or their names are randomly drawn, and they select and unwrap gifts from the pile in that order — with a twist.
The person who receives the number 1 will pick a gift (that is not their own) from the pile and open it for all to see (or models it, reads it, or demos it). During the selection of a wrapped gift, the participant may visually inspect the gifts, pick them up and/or gently shake them.
The person who receives number 2 then chooses a gift and opens it, and then must decide whether to keep it or swap it for the first player’s gift. Each person in order then gets to select a present, open it and decide whether to keep it or swap it for any other gift someone has already opened.
Opening of gifts and swapping takes place until all the presents have been chosen. Finally, the person who picked first gets to choose from all the gifts or keep what he/she has already received. In the end, the gift you are holding is the gift you take home.