Incongruous lexical units and metaphor identification

I picked up this book by Gerard Steen et al. about metaphors at the annual LSA meeting. I need to summarize it for the rest of our team. A key idea in their method for finding metaphors is the identification of 'incongruous lexical units' which may be signals of 'cross-domain mapping'. The 'incongruous' idea is related to work by Charteris-Black and his students.