Downed Fire lets you shoot even when you're incapacitated, but only with your backup weapon, and only after an initial delay. You will have almost no Health while Incapacitated, making it easy for enemies to finish you, so it's less likely that a Medic will be able to revive you.
The usefulness of this ability is heavily reliant on how aware opponents are. At times! most people will assume that a downed enemy poses no risk and will focus their efforts on the immediate area. That being said, if they do notice the player in the typical position for this ability, they'll be able to kill the player relatively quickly.
When downed, with this skill, the secondary weapon will be available to fire. Fire will be quite inaccurate while downed and recoil settles slowly, so high recoil assault rifles and semi-auto pistols are much less useful than SMGs that can be fired on full auto with some degree of control.