Non-verbal casting, or silent casting, refers to the process of casting a spell silently, without having to send the incantation over to the public chat.
This takes advantage of how the game registers spells in the chat. Regardless of what's within a chat message, as long as proper incantation is detected, it will be registered, allowing one to cast confusingly (typing multiple incantations in the same message) or to apply the procedure needed for silent casting.
To cast spells non-verbally, one must attach the incantation to any Roblox chat command (such as /e or /c, the emote and channel commands respectively). Though Roblox will return with no such emote or channel, the game will have registered the incantation, ignoring the command prefix. Thus, the syntax is "/e or /c name of spell".
You can also send your incantations over the system channel. Switch to it using /c system, and your messages will not appear publicly. When using this method, there is no need to repeat the silent casting procedure, and you can type the incantations normally.
Alternative ways of silent casting include whispering the incantation to another player (with the /w command). However, the recipient may still be able to see the incantations. It is also possible to deceive the Roblox chat filter into censoring the incantation by attaching characters to the spell (such as "wappa"). Regardless of the presence of other characters, the spell can still be casted as long as the incantation itself is spelled correctly.
An incantation can also be hidden in plain sight by typing the incantation into a seemingly harmless message. Once again, the incantation alone will still be parsed and it will be possible to cast it.