What is the relationship between checkpoint and wal

Hello:

Sorry for disturbing.
I have one question : Will checkpoint cause wal written happen?

I found the following info at:
http://www.postgresql.org/docs/9.2/static/wal-configuration.html

...
Checkpoints are fairly expensive, first because they require writing out
all currently dirty buffers, and second because they result in extra
subsequent WAL traffic as discussed above.
...

What confused me is that: (checkpoint)result in extra subsequent WAL
traffic as discussed above...

I haven't found any more information can describe it in the "above" of that
page.

What I understand now is:

In Oracle, when checkpoint occur, it will flush data back to datafile.
At that time,
in oracle, even fulsh back to datafile will also cause log writer writing
to redo segment(rollback segment).

But for Postgreql,
I haven't found any information says that:
When checkpoint occurs, before flush to datafile, Postgresql must first
activate some logwriter to write to WAL, And then activate bgwriter or
backend process to write to data file.

Can anybody please give some url link which can say:
That Checkpoint will result in extra subsequent WAL traffic ?

I need some evidence to show to some guys in my team and to my boss.

Best Regards