If you press F3 + N on a Bukkit server with permissions for /gamemode, but without op, your Minecraft client would not allow you to actually change your gamemode. This is because the minecraft client requires to know that you are a server operator.
If you have this plugin enabled, a user with the "F3NPerm.use" permission would get a status update from the server when he joins. This status update tricks the minecraft client to think, that the user is a server operator, without him actually being one. This grants the user no permissions, but it will allow the minecraft client to use F3 + N normally, which would mean, that it would execute a normal command with "/gamemode".