Although he talks a bit about formal specifications and tools for writing and verifying them, his talk has a few really great parts about informal specs.

I'd recommend watching 23:22-30:42, where he gives an example of where an exhaustive semi-formal spec was critical for writing good code.

I'd also recommend 44:00-54:00, which I think is the most broadly applicable part of the talk, where he discusses the need for thinking as writing, and writing specs before coding.