Flash was used for making vivid web pages, but sometimes the size is also matter. To reduce the time of downloading, nested Flash was introduced.

One by one mode
Non interactive Flash is split into several pieces simply, and then be played back one by one. The next piece will be loaded only after the previous one has been played back to the end.

Side by side mode
Interactive Flash is split according to their contents, and related frames are grouped into one piece. Depends on your interaction, the skeleton Flash will load different sub-Flash side by side.

Note: To save them, please refer Saving nested Flash files from a web page.