Sync Point Editing is an extra ability you temporarily give to the Overwrite edit function when you need it. After using it, you turn this ability off. Sync Point Editing allows you to perform an Overwrite edit and at the same time tell Media Composer to sync a specific frame of audio and/or video in the Source Window with a specific frame of audio or video in the Sequence. You use the Position Indicators in the Source window and in the Timeline window to establish the sync point.
For this example, imagine that you have edited a documentary about boxing and you now want to add in a sound effect of a punch to enhance the knockout blow. You have imported a clip of punch sound effects off of a CD, and the clip contains 12 different punch sounds. Some are louder and more forceful than others, so you'll have a choice to make. Further, you want the sound effect to sync with the moment the champion boxer hits the challenger.