Zbrush creates displacement maps for your base model if I'm not mistaken. It takes the base model, turns it into a super high poly model that you use with their tools to texture and alter to your liking. The program then calculates the difference between the base model and the high poly model and renders displacement maps that are applied during render time. This way, you have all the benefits of a high poly model without all of the weight during animation... Is that right??