When altering database SET options such as snapshot isolation,read committed snapshot, we can specify options like No_wait;Rollback Immediate;Rollback after n
Example:
ALTER DATABASE AdventureWorks2012
SET SNAPSHOT ISOLATION ON WITH ROLLBACK IMMEDIATE
Rollback Immediate – Rollbacks all the uncommitted transactions immediately and then alters the database setting.
Example:
ALTER DATABASE AdventureWorks2012
SET SNAPSHOT ISOLATION ON WITH ROLLBACK AFTER 10
ROLLBACK AFTER N – Waits for N seconds and then rollbacks all the uncommitted transactions after that,in the example,it will wait for 10 seconds and rollbacks all the uncommitted transactions and then alters the database setting.
Example:
ALTER DATABASE AdventureWorks2012 SET SNAPSHOT ISOLATION ON WITH NO_WAIT
NO_WAIT – Will try to alter the database immediately and will fail, if it could not apply them.
STANDARD ALTER COMMAND – We can also write the Alter Database syntax without mentioning the above options but behaves similar to NO_WAIT option.
You can read more in this MSDN article.