The trade engine itself does not withdraw or deposit coins. It would be terrible for the exchanges, because it slows the trading down enormously, and also the fee they collect is reduced (or the fee you pay is increased).
When you want to withdraw some coins, the site checks the wallets they own who have enough balance. From one of those (Im not sure how that is selected, but there is probably some optimization there) your withdraw amount is transferred to your address. So you get bitcoin (or whatever altcoin) from a deposit wallet from someone else. This last thing does NOT mean you loose money, as cryptsy knows that you have 1 BTC (or whatever) on the site.
Anyway, the system is still the same as your previous question, and it is not completely clear what you are asking now. The trading works as I answered (and in the comments) over there. I hope the withdrawing is a bit more clear now.