They get special dispensation for being marines, despite the fact that there are better
LD scores out there, to never be hit by sweeping advance.
If they break from combat and are caught they become fearless instead for that round of combat, due to the No Retreat rule.
Fearless models almost (there are a couple of very rare occasions) never fall back (as it's a morale check, and fearless troops are immune to all morale checks) , but if they lose combat they take a number of wounds equal to the amount they lost by, which you can then make saves against.
*edit for clarity*