For knowledge-intensive, fast paced conversations, use whatever recording / capturing devices you can.
In one of my past projects, our professional tech writer used audio recording to capture explanations from our lead developer. Guides he produced were great.
Few months ago I brainstormed design of a particularly tricky module with my tech lead. After we finished, I just scanned all the papers we wrote our thoughts and schemes on and attached it to the respective ticket in issue tracker. That worked like a charm.
Another tip - submit your draft specifications for review as early as possible.
- That tech writer I mentioned above, had a pretty bad habit to hesitate sending for review stuff until it looked good enough to him. Because of that, guides reviews somehow tend to happen pretty close to release when it was too late to fix some bugs discovered in review. If memory serves, we even had to do "doc-only" releases to handle this. Release 2.1.12 - same as 2.1.11, but with updated guides. :)