The Otherworld, she explains is composed entirely of energy. Our psyche travels there and uses our imagination to decode signals and energy forms in much the same way that our eyes use light to decode shapes, colours and textures for our brains to assimilate.
The entry of the shaman into the Otherworld will alter the tides of the physical world so that a shaman can learn to create changes which will have real effects in the physical world.
Shamanica is clearly written and gives many techniques for the modern shaman in an enjoyable and easily understood form.