Ok, this is a pointless feature for 999 out of 1000 flashers, but for me and the other 1 out of the thousand it CAN be useful for publishing stuff for flash component sites that dont yet support CS4. The ideea is that you can make use of FP10 features in files saved/published by Flash CS4 as CS3, for FP 9. This might also work in some cases for CS3 files published for FP9 by CS3 (I can think of properties here, but dont have a Flash CS3 to test.
Things go like this, you have an external preloader, for which you could even not supply the source code, published to FP10. Lets assume you want to use 3d stuff from the FP 10 api in your component but of course Flashden doesnt let you submit CS4 files. You have this class A extends MovieClip that really really wants to change rotationX of itself. MovieClip is one of the few classes that are dynamic by default, but if you inherit it you still need to specify that your child class is dynamic (ie. dynamic attribute is not inherited), so you do that. After that you decide that you can add a new property called rotationX (which is not present in FP9 api but the class being dynamic, flash thinks it’s your property). You then publish the file to FP9 and run it. Of course rotationX is not taken in account, since FP10 loads this file as an FP9 file which has no access to the FP10 apis. Now what happens if you load this file as a child swf in the FP10 published loader ? You guessed it, rotationX is interpreted as a FP10 DisplayOjbect property. With some tweaks this can possibly work to methods also and to a certain extent I believe, as stated earlier that it can work for actual CS3 files also.
This was probably a thing you didnt knew about flash!
dw