TFLint rules are provided by plugins. The plugin is launched as another process and communicates over RPC. Inspection requests and configuration file fetching, expression evaluation, etc. are performed by bi-directional communication, and the host process and plugin process act as both a server and a client.
The plugin system is implemented by go-plugin. Since it uses a net/rpc based implementation, it uses hashicorp/yamux for communication multiplexing. See also the go-plugin architecture description.