Scenario using pg_rewind

Hi,

I have a question of how to use pg_rewind.
I see the manual. It says bringing an old master server back online after failover is typical case.

manual:
A typical scenario is to bring an old master server back online after failover, as a standby that follows the new master.

I came up with an another scenario where user uses a synchronized old master as a new master.
If so, pg_rewind need to copy WAL from a source server.
Is pg_rewind assuming to use the old master as a new master?

If pg_rewind is used only for a standby server installation,
pg_rewind does not need to get WAL from source server, does it?

regards,

--
Kato Sho