Graveyard compactions, when do they occur?

[Erik Forsberg]

Hi!

I was trying out the "truncate" command in cassandra-cli.
http://wiki.apache.org/cassandra/CassandraCli08 says "A snapshot of the 
data is created, which is deleted asyncronously during a 'graveyard' 
compaction."

When do "graveyard" compactions happen? Do I have to trigger them somehow?

Thanks,
\EF