"Rapid learning" and troubleshooting methods. The process for both of these is nearly identical, although some details differ. There are several books by Steve Litt that go over these and I found helped me troubleshoot problems faster than most of my co-workers, but the same principles helped me to understand a new code base much faster.
Available books by Steve Litt: http://www.troubleshooters.com/bookstore/index.htm
A nice book package, I recommend these to every developer I meet: http://www.troubleshooters.com/bookstore/recession_package_a...
His website isn't pretty, but the content is golden.