The process by which ions and neutral atoms or molecules in interstellar gas travel at different speeds in the presence of the interstellar magnetic field. A gas cloud that is collapsing to form stars can detach itself from the interstellar magnetic field through this process, so avoiding the build-up of magnetic pressure that would otherwise halt the collapse. Provided that the collapse is slow, the neutral atoms and molecules can slip past the magnetic field lines in the gas. In this way the bulk of the cloud collapses but some of the magnetic flux leaks out.