Question about how Damage works with "FnP"

While I'm reading the rules and the new Ta'u index, I had a question about how the Stim Injectors (which is essentially a 6+ FnP) works with woulds/damage. Specifically, do you roll one dice for the wound and then let the damage go through or do you roll one dice for every damage done?

E.g. A lascannon hits my Stormsurge equipped with Stim injectors. The lascannon wounds and rolls a 5 for damage. Does the stim injector stop all damage on a 6 or do you roll a 6 for each damage done and that one damage is prevented on a 6?

The wording is "..each time a unit suffers a wound or a mortal wound. On a roll of 6, ignore it."

If anyone could shed some light on this, I would appreciate it. Sorry if this is something really easily answered, but I haven't come across a decent explanation.

Thank you!