Regarding roundtrip for calculated attributes

All,

I am following the tutorial and I didnt understand the content conveyed regarding roundtrip.

http://help.sap.com/saphelp_nw70ehp1/helpdata/en/15/6de9403070712ae10000000a155106/content.htm

Here I was asked to create an action with name Roundtrip and assign this to onEnter. But I didnt provide any implementation for onActionRoundtrip method.

How it is able to calculate the value for the field?

And I dont understand the last sentence regarding onLeadSelected. What will happen with assignment of Roundtrip action to this event?

Thanks.

Nag