Off hand is, in your case, your left hand. When we had to do firearm qualifying for the military department, we had to pass with off hand shooting. This is most prominent if you are behind a barricade (wall or some other kind of structure for defense), and you can only use your off hand to aim. IF you are right eye dominant, and you need to go to your left hand to fire your weapon, if you try to use your right eye, the bad guy is going to get an easy head shot since you have to cross over your head on the weapon.
If you train yourself to use the other eye (left eye for left hand, right eye for right hand), then you are a much smaller target. I did a lot of practice with my left eye, but still found if I was just standing looking at a target, I would still use my right eye with my left hand. But, behind a wall, I would always use my left eye to keep myself from exposing as much of myself.
Thankfully, I never had to use this training, but it's good training to fall back on when the fecal matter hits the aerial rotating device. I'm sure, even though I have been out of law enforcement for quite a few years now, that I would default back on my training. Which is why I think anyone that wants to carry a firearm should have to go through a training class and qualifying sessions at least once a year. I am a firm believer in the 2nd Amendment, but I do also think there are some that do not need to be carrying a firearm as they couldn't hit the broad side of a barn from INSIDE the barn...