A 24″ plus chain shank can be run over the nasal bones through the halter rings. A shorter one will pull the halter over or bind in the halter ring. It is used as an amplifier of signal for horses that are accustomed to pushing through pressure applied to a halter.
It must be used correctly or it amplifies bad responses and can be dangerous. Timing and release are the two critical factors. If the chain binds, then pressure is not released and the horse will respond negatively. I do not recommend using a lip chain unless the operator is very experienced and other horsemanship options have been tried.