First curtain sync fires the flash at the beginning of the exposure. Second curtain sync fires it at the end. The difference, on a stationary subject in a dark room, is invisible. The difference, on a moving subject with any ambient light at a shutter speed slower than 1/60s, is the difference between a motion blur that trails behind the subject and one that leads in front of it. The first looks like the subject is moving backward.

This image says almost everything I usually struggle to explain with words. A photographer leans forward, arm stretched out as if reaching into the scene, camera pressed close to the face, body slightly twisted to find the angle that feels right. The built-in flash is popped up, but it’s not firing, and that tiny detail is kind of the whole story. The moment is quiet, focused, almost intimate, and that’s exactly the kind of moment a built-in flash loves to destroy.