What he means is, when the trade is made, mtgox would need your permission to do the actual trade. The bitcoins would then go into a new joint custody of mtgox and the guy who just bought them from you.
Ahh right, I see what he meant. Though if that's case, what's the benefit of mtgox having custody over the traded bitcoins? If we could fill all the missing details in this scenario (we cannot because that's not how real-time exchanges work) then giving control over the bitcoins just to the guy who bought them seems to be enough, and joint control with mtgox would be useless?