External representations serve as visual aids for problem-solving and creative thinking. Past research has enumerated some of the features of external representations that enable this facilitation. We have questioned how and why architectural design sketches facilitate exploration of design ideas, by conducting protocol analyses of designers’ reflections on their own sketching behavior. Our previous analyses of their protocols revealed that skilled designers, once they shift attention to a new part of a sketch, are able to explore related thoughts more extensively than novice designers. How do they keep focused on related thoughts? What are the driving forces for successive exploration? We examined the types of information that expert and novice designers considered during and between chunks of related thoughts. We found that focus shifts driven by consideration of information about spatial relations led to successful exploration of related thoughts. We relate these results to some aspects of facilitation by the externality of sketches.