Basically what that means is pull the trigger without any side to side pressure. Also, just because the finger is at an angle on the trigger, doesn't mean it's being pulled at an angle. Basically - pull the trigger without moving the sights and you'll be fine.
I wouldn't stay away from exerting a strong grip with only your middle finger. There are some great shooters who prefer a strong support hand grip and a relaxed strong hand grip. Equal pressure in both is also popular. I personally death grip the shit out of mine, but I wouldn't recommend that to most people. Experiment on your own to figure it out, but I would urge you to not think in terms of pressure of each finger, but pressure of your grip as a whole.