What function does Mt. Gox's "Open Transaction" withdrawal option perform? When withdrawing bitcoins from Mt. Gox, there is an option with a checkbox next to it titled "Open Transaction (6 confirmations)".

What does this do to my withdrawal?

I'm thinking this is probably a mislabeling for an option that ensures that even if the receiving address is for another customer account at Mt. Gox that the transaction will be sent through the blockchain.  Otherwise, if those transactions are handled internally without appearing on the blockchain then it is difficult to prove that a bitcoin payment was sent.
I just asked on the #mtgox IRC channel, and was told:

06:36 &lt; dooglus&gt; what does the "Open Transaction (6 Confirmations)" option
                 do when withdrawing BTC?
06:48 &lt; Cory&gt; If the Bitcoin address you're withdrawing to is a MtGox address,
              it will use the network and blockchain anyway.
06:49 &lt; Cory&gt; Otherwise, if that option is not checked, they will only internally
              transfer the funds.
06:53 &lt;@MagicalTux&gt; and instantly


So there you have it.

I suggested they should label it differently:

07:13 &lt; dooglus&gt; it would be better to use some different text for that option -
                 like "use bitcoin network even for transfers to mtgox deposit
                 addresses" for instance
07:19 &lt;@MagicalTux&gt; dooglus: it use to be something like that, but people were
                    complaining it was not clear enough