what is snapshot controlfile and when we can make use of it??

RMAN needs to get a consistent view of the control file when it sets out to make backup of every datafile. It only needs to know the most recent checkpoint information and file schematic information at the time backup begins. After the backup starts, RMAN needs this information to stay consistent for duration of the backup operation; in other words, it needs a read consistent view of the control file. With the constant updates from the database, this is nearly impossible - unless RMAN were to lock the control file for the duration of the backup. But that would mean the database could not advance checkpoint or switch logs or produce new archive logs. Impossible.
To get around this, RMAN uses the snapshot control file, an exact copy of your control file that is only used by RMAN during backup and resync operations. At the beginning of these operations, RMAN refreshes the snapshot control file from the actual control file, thus putting a momentary lock on the control file. Then, RMAN switches to the snapshot and uses it for the duration of the backup; in this way, it has read consistency without holding up database activity.

ie , When RMAN needs to resynchronize from a read-consistent version of the control file, it creates a temporary snapshot control file. RMAN needs a snapshot control file only when resynchronizing with the recovery catalog or when making a backup of the current control file.

The default value for the snapshot control file is platform-specific and depends on the Oracle home. For example, the default filename on some UNIX platforms in Oracle Database 10g is $ORACLE_HOME/dbs/snapcf_@.f. Note that if you have a flash recovery area configured, the default location for the snapshot control file isnot the flash recovery area.

You can see the current snapshot location by running the SHOW command. This example shows a snapshot location that is determined by the default rule:

RMAN> SHOW SNAPSHOT CONTROLFILE NAME;

CONFIGURE SNAPSHOT CONTROLFILE NAME TO '/oracle/dbs/snapcf_trgt.f'; # default

This example shows a snapshot control file that has a nondefault filename:

RMAN>  SHOW SNAPSHOT CONTROLFILE NAME;

CONFIGURE SNAPSHOT CONTROLFILE NAME TO '/oracle/oradata/trgt/snap_trgt.ctl';

Setting the Location of the Snapshot Control File

Use the CONFIGURE SNAPSHOT CONTROLFILE NAME TO 'filename' command to change the name of the snapshot control file. Subsequent snapshot control files that RMAN creates use the specified filename.

For example, start RMAN and then enter:

RMAN>CONFIGURE SNAPSHOT CONTROLFILE NAME TO '/oracle/oradata/trgt/snap_trgt.ctl';

If one RMAN job is already backing up the control file while another needs to create a new snapshot control file, you may see the following message:

waiting for snapshot control file enqueue

Under normal circumstances, a job that must wait for the control file enqueue waits for a brief interval and then successfully retrieves the enqueue. Recovery Manager makes up to five attempts to get the enqueue and then fails the job. The conflict is usually caused when two jobs are both backing up the control file, and the job that first starts backing up the control file waits for service from the media manager.

what to take care in RAC environment ? 

Identical copies of the control file must be maintained on every node that participates in the RAC backup process. Therefore, each node must have an identical directory location to store a snapshot of the current control file ,So it is necessary in RAC environments that the snapshot controlfile is located on shared storage. Otherwise your RMAN backup may fail.For this reason you can put the snapshot controlfile into ASM .

For example 

RMAN> configure snapshot controlfile name to '+FRA/snapcf_oralin.f';  

When it will get generated? 

Basically the snapshot controlfile is created when oracle needs a "read-consistent version" of the controlfile.In my case I have a catalog database which is synchronized at the beginning of the backup so the snapshot controlfile is created at the beginning of the backup.If you backup with rman without a catalog it will be at the end of the backup.

Is the snapshot control file ever needed during recovery? 

Short and right answer is "no". 

Read the definition of the (RMAN) "Snapshot Control File" below.

Generally I would consider the (RMAN) "Snapshot Control File" worthless as it knows *nothing* about the backup (sets) currently being completed/made 

Meaning the last backup metadata is not *in* the "Snapshot Control File", as it was "snapped" before the back started/completed.

*HOWEVER* it is a copy of the controlfie at the beginning of your current backup and while it knows nothing about the current backup, it "still" has knowledge/metadata about all previous RMAN backups (not deleted, marked missing or obsolete by RMAN).

I can invision a scenario where I lost *ALL* controlfiles on disk and on tape *AND* I do not use a RMAN Recovery Catalog. I would try and use the last known available RMAN "Snapshot Control File" to restore my database from an *older* (than the Snapshot Control File) backup and recovery my database.