Please give me a simple explanation in this.

Fave Photog wrote:

It's not an issue of what happens DURING the micro-second that the flash fires and 'freezes' the subject. It's what happens AFTER.

Using Front Curtain Sync, the motion blur created AFTER the flash fires gets imprinted on TOP of the 'frozen' subject, thereby causing the edges to become blurred and also creating all sorts of unwanted ghosting on the subject. Please see the attached image titled 'Front Sync'.

Using Rear Curtain Sync, the opposite happens - the subject gets imprinted on TOP of the motion blur that occurred BEFORE the flash 'froze' the subject, thereby causing the edges to remain sharp, distinct, and with no unwanted ghosting. Please see the attached image titled 'Rear Sync'.