To give an example of its use:

When you need to update your CPR you take what’s called a challenge course. In the CPR Challenge you show the instructor that you know how to do CPR on your own, without help. If you need help, or you can’t remember something on your own, then you will not pass the challenge and will have to take the full CPR course all over again. Is it really fair to fail someone on their first mistake though? No, it’s not. So they get about three chances:

First Mistake – They’re given some help, kinda like hints. (but not on something big and important)
Second Mistake (strike 1) – They are sent to their book; with a choice to either look it over real quick and come back, or to read over their book while someone else takes their turn.
Third Mistake (strike 2) – They are sent to their book and are now at the end of the line.
Fourth Mistake (strike 3) – They are out. If they can’t get it by this time, then they cannot be trusted to remember it in case of an emergency.

I know it looks like four strikes, but it’s three. The first mistake is normally something small anyway.

I could explain each of those strikes in the terms of a CPR challenge, but to better explain the three strike rule in terms that many people can understand, I will relate it to baseball.

When you are very young you are first taught how to swing a bat. You probably first learned in T-ball. The ball sits on the ‘T’ and you just swing away. Sooner or later your bat makes contact. The next step is having a ball tossed to you. Now you have to learn to hit the ball out of the air. Then the ball is pitched to you, first by coaches and then by other kids your age. You have to learn when to swing. You swing too early, you miss. You swing too late, you miss. You swing too high, too low, at an odd angle, etc. you will mis the ball. You have to have near perfect timing. This is why you practice. Either by going to the batting cages or by having someone pitch to you. You practice hitting fast balls, slow balls, curve balls, and sliders. You work hard to learn all you need to know in order to step up to the plate and smack that ball into the outfield. You practice and practice and practice and now… it’s game time.

Your team mate is up to bat and you are "on deck" waiting for your turn. As you wait, you try to get your timing right with each pitch. Your team mate gets a hold of the ball and sends it deep into right field. He manages to make it all the way to third base! Amazing… and now it’s your turn. You set yourself in the batters box and get ready. You grip the bat tight and repeat to yourself, "keep your eye on the ball, keep your eye on the ball…" The pitcher winds up and throws the ball right through the strike zone. You swing and wiff. Strike 1.

Your timing was off; you swung too early. Should you be sent back to the bench? No. Hey, you’re only human, right? Everyone makes mistakes, and to give someone only one chance to get it perfect is just ridiculous. So you are given a chance to make adjustments. Last time you swung too early. So this time you must wait a little longer before you swing. The pitcher winds up. He makes the pitch. You swing. Strike 2.

You missed again. How? Well, you overcompensated. First time you swung too early. This time you swung too late. You waited too long. So now you finished, right? Wrong. Anyone can see that if you didn’t get it the first time and were given a chance to correct your mistake that it would be easy to over-correct. So you are given one more chance. Now you have both ends of the spectrum. Too early and too late. You remember what "too early" felt like. You remember what "too late" felt like. Now is the big challenge. Can you find the center and swing at the right time. Can you hit that ball deep and get your team mate on third in to home, and score a run. The pitcher winds up and sends the ball straight through the strike zone.

So you see, three chances are plenty. The first chance you end up with a result that is wrong. So you know not to repeat that result again. The second time, unless you get it right, you can end up with another incorrect result. There are only so many results that make sense concerning the question or challenge. Now you know of two possible results that will not work. If you can’t find the right solution on the third try, any more guessing will be just that; guessing. The more times you guess, the more of a chance you have of getting it right, but if you get it right it’s possible that you are just lucky for actually finding the right answer. It doesn’t prove that you know the answer. It doesn’t prove that you have the proper knowledge of the subject. When we are talking about CPR, you need to really know your stuff. You cannot come across a victim and begin to think about what could be the next step. You have to know. So when being challenged, you have prove that you will know what to do in case of an emergency.

We can’t have people out there certified in CPR who don’t know what they are doing. People’s lives are at stake. The three strike rule allows a candidate ample tries to get the right answer without relying on dumb luck to produce the correct answer. This way we end up with a person trained in CPR who knows what to do.