I like "Know the Situation" as a reminder. That means for example if there is a runner on third in a close game with less than 2 outs and you have 2 strikes you can give extra focus to just hitting the ball on the ground to the right side. Protect the Plate is really a verbal reminder as well. I think it actually says the coach is very worried the batter is not in tune enough and is the type of hitter to watch a third strike. It's kind of like a nervous wish by by the coach that the batter won't strike out and somehow shouting it out will impact the batter.
Personally, I don't particularly like this saying. I think it puts the batter in a defensive frame of mind. I much prefer them to stay in an offensive frame of mind and attack anything that's close.
1. With zero strikes a batter is looking for a pitch in their favorite 6" x 6" zone. My dd prefers mid thigh, middle-in.
2. With one strike a batter is looking for a pitch in the rules stated strike zone.
3. With two strikes a batter is looking for a pitch from river to river, and armpits to mid shin (expanded zone). Foul off the junk and find a pitch to drive.
Until I have two strikes I don't give two Hershey Kisses about the strike zone. The mentality is that I am a predator of the ball, and my criteria for completing my swing is that I can get long-through the ball. It isn't until I have two strikes that I care about the strike zone ... now I am in "battle mode" and I open up the umpire's strike zone slightly. If you want to call that "protect the plate", I'm fine with that ... me, I call it "battle mode".
Protect the ump's strike zone. In more advanced levels you better know the umpire's strike zone. His zone may have just changed too, if it's near a time limit and he wants to go home.
As others have said, you lose ability to be picky. Pitch recognition = pitch selection.
If you take a pitch you better look confident. Meek or disinterested body language will generally expand the zone in the umps eyes by 1 or 2 inches.