what is user-extension and how to use it ???

hi folks,
I am trying to understand how to use user-extenstions with selenium-rc but not able to figure it out. I created a use extension and run my selenium rc by giving it as a parameter.

and then i try to call the method in selenium rc but it shows me error that this particular method is not present under selenium object. which is true. I mean i haven't write down any method inside default selenium class.

so how to use the user-extension inside selenium rc ??

after creating u r own user-extensions.js u can use it while starting the selenium server. it

Ex:
java -jar selenium-server-standalone-2.20.0.jar -ensureCleanSession -trustAllSSLCertificates -port 4444 –userExtensions user-extensions.js