What an overworked phrase! Catchy, but overworked. Were not talking about when to take a Viagra or Cialis. Professional photographers have known for decades the kinds of photos they capture when the serendipity springs up before their eyes. They capture the moment because they press the shutter at just the right mille-second. Sometimes such moments can be planned, and sometimes they cannot. Sometimes they are just there, and you have to snap it without thought for composition or lighting or any of the other elements we know are important for quality photography.
For example, you can plan for a photo full of the energy of the win when a person or a vehicle crosses the finish line. Or when you know something spectacular is about to happen, like the birth of a baby, having a camera ready for that first moment of life can result in a breathtaking picture. Such life moments can be captured if you are ready with your skill and talent and equipment. Your education, training and experience are as important as the serendipity in such instances.
Equally so, there is something to be said for the magic of the moment, captured forever on film or a CF card. that moment when...