The call Trade the Wave means that each pair of dancers who are facing the same direction Trade with each other. As with any Trade, they will end up in each other's spots. But because everybody in the wave will be doing this at the same time, there won't be anybody standing still to mark the spots. So it is very important for each dancer to locate that other spot before starting to move.
As the dancers trade, they pass right shoulders, and then each of them walks into the other's starting spot, but facing the opposite direction. Meanwhile, the other two dancers are doing the same thing with each other.
Trade the Wave from a right-handed wave produces a left-handed wave. Note that each end dancer moves to a center position, and vice versa.
If Trade the Wave is done from a left-handed wave, the result is a right-handed wave. But they still pass right shoulders.