Being a Sitter in the Hall for a time means that Farnah is also a high ranking sister and a strong channeller by Aes Sedai standards. Thus she has the strength needed to use the travel weave.
The Sitters try to hold a secret meeting without Egwene, discussing ways temper the Amyrlin's power when a declaration of war is called. Farnah voted for a proposal that would allow only Egwene to deal with the monarchy of the world, which includes Rand as he is King of Illian, while the Hall would take over prosecution of the war and deal with Gareth Bryne's army. In that same meeting, Farnah voted in favor of the new rules and procedures in the Hall, which included temporarily replacing a Sitter when she is absent from the Hall and that no meeting of the Hall can commence without all Sitters and the Amyrlin Seat being present.