Can't get JJsploit to work.

This is called Filtering (Or "FilteringEnabled"). It is a network model introduced in Roblox in approximately 2014.

Its primary reason of use is that it prevents rogue clients (Referencing cheaters, mainly) from altering games in ways that are not allowed / pre-programmed by the developer. In the pre-2014 development environment (Filtering is disabled), Roblox as a game engine is structured around having the clients connected to one central server - the clients can run scripts locally that were pre-inserted by the developer to alter the world around them directly, this meant reduced latency - but at the same time introduced a problem, which was cheaters. Using cheats, a player could 'execute' or 'inject' scripts into their client, that could manipulate the world around them, and it would be broadcasted to all of the players, meaning that it was easy to do pretty much anything in games. The 'Filtering' networking model ensures that the environments housed in the clients and server had only one way replication (replication refers to a change happening on one side, then being mirrored to the other) - from server to client. The only way that clients communicate to the server is by having only certain physics actions replicate (such as character movement), and most other actions are handled by what's called 'Events' - which are objects that the client can send data into, and the server can then receive the data, process it, validate it, and do what it wants with it. You can read more about it here and here.