Say I created a branch in perforce of our codebase. Here is the branch spec:
//depot/code/main/... //depot/code/branch/...
Then, in the branch,
You can add '-3' switch to use a new engine for integration, which will detect target files that have been previously moved with 'p4 move', and automatically 'retarget' itself to follow those move operations.
p4 integrate -3 //depot/code/main/... //depot/code/branch/...
will integrate your changes in //depot/code/main/a.txt to //depot/code/branch/b.txt.
This is the 'undoc' feature in current 2010.2 release, but will be the default behavior in the upcoming 2011.1.