Locking down legacy code is one of the major problems when trying to work on existing code. Approval tests facilitates this in a couple of ways.
1) producing lots of results
2) verifying large results
http://blog.approvaltests.com/2008/10/approval-tests-pictures-worth-1000... [Bottom of the page]
Here is a video of me locking down the code from LegacyCodeRetreat.