Another way of thinking of it is that the dancers are trying to do a Step to a Wave, but some of the dancers need to turn around in order to do it. Only the leaders need to turn around. The trailers, who are already facing the other dancer in their tandem, don't turn -- they just Step to a Wave.

Each dancer makes this decision individually, so it can be done from any combination of leaders and trailers.

Left Roll to a Wave is the same thing, but in the other direction. The leaders will turn around halfway to their left, and everyone then steps to a left-handed wave.