How does SkillClient's BungeeHack work?

I'm just curious on how a player spoofs their IP using BungeeHack on SkillClient. I understand how a player could just run their own BungeeCord server and link it with someone else's server to grief it with an admins account (The reason for plugins such as IPWhitelist and OnlyProxyJoin), however I don't understand how they can change their IP to whatever they want?

I've done some testing and I was able to connect with anything as my IP.
Letters? Yep.
Emojis? Yep....

How does SkillClient's BungeeHack work?