I also am running OSx 10.5.6. Th4 X key, if it is pressed during startup, is supposed to start you up in OS 9 if it is on the same disc. From the small bit you entered into the forum, I'm not sure if you are talking about a specific program, or if you are wanting to start up in OS 9.
I tried holding the X key down on my mac but nothing happened, because I don't have any f the classic environment installed at all. Apple quit with support for the classic (OS9) environment with the intel machines.
If you have an older machine that has OS9 installed in a separate partition, you might go to the system preferences icon on your dock and click on it, and then click on the "Startup Disk icon" which will give you the ability to switch between your operating systems.
I hope I've helped
