Most private servers use a .ini file to tell the client which GRFs to load.
Here is the core mechanic that advanced modders exploit: ragnarok 0 delay sprite grf top
This is the origin of the "0 Delay Sprite." You aren't cheating the server's logic; you are blinding your own client to force it to send packets faster. Most private servers use a
For players utilizing these files, the gameplay experience changes drastically. Skills that traditionally have long, clunky cast animations suddenly become fluid. This is the origin of the "0 Delay Sprite