How does unique index work?

Strange question. But can somebody shed some light on the internal
workings of an unique index?

1. Insert 1 row into a table with an unique index on it.
2. Old data and index blocks that the insert statement touched are
move to rollback segment.
3. ORALCE enforces the uniqueness due to the index. How? By doing a
full scaning on the table column and the index?
4. Insert another row into the table. How does ORACLE enforce the
uniqueness? ORACLE does not need to look up the rollback segment to
check for the uniqueness, right?

Thanks!

John