There are 3 times this exploit can be used.
1. Announced in-game server reset
2. Weekly Wed. Mainenance
3. Random server crashes (These are hard to predict, since they are well, random. I don’t bother with these but I have heard a pretty safe time to start the process is after someone complains of hp/mp not regenning due to server issues – generally means a crash is forthcoming).
Basically this uses the rollbacks which NC states don’t happen on every server reset. If you leave your char logged while the server resets it WILL get rolled back by a couple minutes. So to safely dupe you enchant your item ~2 minutes before the reset (time it based on inagme announcements, these are generally very accurate). You have about 1 min to get the enchants you want done. If you succeed, log out using the normal commands and your new enchants should be saved. If you fail and loose the item, just stay logged and when the server comes back you will have your item back.