The following is nothing more than some extension of the oracle metalink document:
[ID 740029.1] Step By Step Guide On How To Configure And Test Client-Failover For Dataguard Switchover And Failover
That is, the article does not only present the cookbook how-to of its anchestor but also examines what happens in the database and what the clients experience will be concerning a so called transparent application failover (TAF). The scenario has been tested on an oracle 10.2.0.3 on windows, please note that the metalink document points to a some more applicable technique available with oracle 11gR2 (Client Failover Best Practices for Highly Available Oracle Databases: Oracle Database 11g Release 2).
Client-failover for dataguard, away from other failover scenarios, essentially aims at using a general tnsnames entry against some server-side database endpoint, no matter what server-side database instance is currently running in a dataguard primary role. This is actually the transparency in a transparent application failover where the application, client here, does not have to care to switch its network configuration in any way (some simple implementation would be, not uncommonly, to provide some
tnsnames.ora.stb and rename the files) during a failover or switchover.