A "decisive moment" is just a single moment that tells a larger story. It's not magic. There's no formula for it. It's just a moment where enough things come together that it's easy for the viewer to see the greater picture that extends beyond the image presented.
A "decisive moment" can be a still, a video clip, a still from a video clip, hell, it can even be an audio clip. If you romance it, you won't understand it. The key to capturing the "decisive moment" is having the sensitivity to understand when that moment is happening and what the moment means. Then you can figure out how to capture it.
Take "Puddle Jumper". What does it say? What's the greater story behind it? Why does it resonate? Why do you connect with it? To me, it's a story about modern life. A man who is obviously in a hurry to get somewhere. He's stressed. You sense that he's late for something by his long stride. You fear for his comfort and appearance, as you can see he's about to sully his clothes. You even fear for his health, as he's literally taking a leap of faith into the unknown (there could be a hole at the end of that ladder under the water). His image is blurred. He's a featureless individual, so he can represent all individuals. He's a nobody and an everyman. That stressful, yet nearly meaningless existence is what so often defines life in the post industrial revolution. We are treated as machines by the machines that run society. But we are not machines. We are people. We are scared. We are fragile. And most of all, we are just trying to survive and make ourselves valuable in a time when the value of human capital is at an all time low.
Did HCB plan all of this out ahead of time? Did he even envision all of that out after he took the shot? Probably not. But he did have the presence of mind to recognize the aesthetics of the symmetry that the water created. He did have the sensitivity to recognize the tension created by the decision to move forward and get wet or retreat and stay dry. And he did have the experience to know that the dynamism of a body in motion would enforce that tension. And he knew it was similar situation we all have faced. A universal truth, if you will. And that's why it was "decisive".