New Session

I need to call a transaction in a new session when ever a user clicks a customer in basic list.so could any one tell me how to create a new session(external session).i have tried to see the code that is written by SAP to create a new session, but i could'nt find that button in gui status of any screen in SAP and even there is no code in PAI.so could any one tell me how to do that or else please tell me how to see where the code is written for that.

hope to see some replies on this..

bye

yogi raj