As Darrin said. Voice commands. That's what I've been told and im slowly learning with my own mare. That's how she was trained. And I've seen other TW riders doing the same.... voice and reins.
I know one fella that has two walkers and he trained them both to different sounds. So each horse has its own learned commmand to go into its running walk, and then its own command for galloping.
My mare a simple clucking sound and she will go into her running walk with out any other action. Get her going and the occasional cluck to keep her there.