If the Recklessness skill is lower than your attack skill (as determined by your equipped weapon) then the damage rating is reduced proportionately. The damage rating caps at 10 for trained and 20 for specialized so there is no reason to raise the skill above your attack skill. The damage rating adjustment for incoming damage is also adjusted proportionally if your Recklessness skill is lower than your active attack skill.
While active your non-critical outgoing melee and missile hits add this damage rating to your total.
While active non-critical incoming attacks from all sources are likewise increased.