Under the "Wet Foot-Dry Foot policy", any Cuban migrant who wishes to be an American citizen can try crossing the border. If they are caught in the water before they get to the US they have "wet feet" and are sent back to Cuba. If they are able to cross they border they have "dry feet", and are able to legally apply for a US citizenship.
Example: many Mexicans from the north emigrate from Mexico to southern US. Because of this, northern Mexico is experiencing a labor shortage. Thus, Mexicans from southern Mexico are moving North.