All the dancers drop hands, walk forward, and take hands again to form two parallel waves. The new waves have the same handedness as the original center wave. If the original center wave was right-handed, both new waves will be right-handed.
For the dancers who start in the center wave, stepping to the correct hand is simply a matter of walking straight forward and making the same kind of wave from which they came.
The dancers who start in the outside couples can each "follow a back" of the dancers in front of them. If they are looking at the back of the dancer immediately in front of them, they also can just walk straight forward. If they are directly facing someone who is facing toward them, they must individually veer out a bit to become ends of the new wave.
So if Extend is done from a left-handed wave, the result will be parallel left-handed waves.
Of course the dancers who start in the outside couples can also just look at the center wave in front of them, and make the same kind of wave.