[joshontheweb]

You can ‘tumble’ bitcoins to obfuscate the true source and destination walets

[dogma1138]

The only thing that tumbling does is prevent a direct link between 2 possibly illicit transactions.

Unless the tumblers are use for mostly legitimate transactions they are useless and being linked to to one will raise a flag if you are under investigation.

[jhanschoo]

But Tumbling isn't an inherent characteristic of Bitcoin and requires trusting a third party.