Spirit walking is the ability to enter another being's body whilst the user retains their own mind. People who can Spirit Walk are known as Spirit Walkers. It is an extremely rare ability, said to only be given once every thousand years, but apparently in the even more distant past it was far more common. Torak is the only Spirit Walker mentioned in the series.
While the name- and clan-soul may leave one's body (the world-soul stays in your body), and this happens especially often in times of anger or stress. During a spirit walk, however, you can control your souls. A larger, more powerful creature such as an Ice bear is far more difficult to control, although Torak helped such a creature escape in Soul Eater. Even a tree can be spirit-walked into, and when one does so, the entire forest-nexus can be accessed, allowing for communication over long distances. This use for spirit-walking was only used by Torak in Oath Breaker.
Torak discovered this ability accidentally while talking to a pack of wolves, then shortly became a terrified capelin while trapped in a seal net. He later accidentally entered a seal, and it was not until he was informed by Tenris about the use of this power did he know what exactly this phenomenon is. In Ghost Hunter, Torak's entering of Eostra's body is crucial in helping him win against the last of the Soul-Eaters.