Please give me a simple explanation in this.

Fave Photog wrote:

The purpose of REAR curtain sync is to allow for using SLOW shutter speeds, so that anything lit by ambient light will be properly exposed, AND so that if the SUBJECT of the image is MOVING, that it will be 'frozen' by the flash and not be blurry like it would be when using FRONT curtain sync.

With FRONT sync, the flash fires as soon as the shutter opens, therefore any motion by the subject AFTER the flash fires and BEFORE the shutter closes will cause the subject to be blurry. With REAR sync, it fires just before the shutter closes, thus 'freezing' the subject immediately before the shutter closes, so there is no possibility of recording any further subject movement.

Now, Auto-ISO will always choose the lowest ISO setting needed to achieve proper exposure. That's why it consistently chose ISO 200 when you had the flash connected. The FLASH didn't need any higher ISO than 200 to insure that the dominant (foreground) subject(s) was/were properly exposed.

Using REAR sync does not lock ISO at 200. As explained above, that is the ISO that the camera determined was sufficient to properly expose the foreground subject(s) when using the flash. Otherwise, when you set the ISO to 3200, the camera should not have functioned, according to your mistaken assumption(s).