In the X-wind model the magnetic field of a young star interacts with the magnetic field of the circumstellar disk to produce a gap between the star and disk. As gas spirals inward through the disk, it divides at the inner disk edge (the X-region) into two streams. A high angular momentum stream is flung away along the rotating magnetic field lines of the disk (the X-wind); the low angular momentum stream falls onto the star and helps build its mass.

Image source: http://www.psrd.hawaii.edu/Dec01/Oisotopes.html.