Most good SEOs have no problem explaining how they do what they do. Good SEOs want their clients to understand what they are doing. It helps to build trust.
Building trust with clients is important so clients will know the SEO firm is working hard to restore their rankings if search engines change their algorithms and cause rankings to drop temporarily.
Words like doorway, hidden, or automated are usually a bad sign unless you are willing to take large risk. While most do not, some industries will require aggressive sales techniques. Anything that is “just for the spiders” is usually a bad call if you are trying to build a long-term brand.