It is a recurring theme in photography. Ever since Cartier-Bresson coined the phrase the "decisive moment" has been pursued by photographers around the globe. It's most obvious form, is the unplanned, but epic event that gets captured on film because some photographer is looking through the viewfinder at that very same moment, and presses the shutter at the right time.
While nice, not all "decisive moments" have to be a photojournalistic flash. The decisve moment -in its purest form- is present in every single shot we take (or that we could have taken) and it is that moment where lighting, exposure, motion and expression are glorious and capture the moment. Even in studio situations you have a decisive moment, the lighting and the exposure may be locked down, but not so the expression and motion.
Lights are set, shutter speed, ISO and aperture, preset and frozen in "M" position. But there she goes, and she turns around just a bit, in a perfect moment. I press the button.