A typical use case of this is when you are working on a "feature" that has a bigger risk of instability. In such a case, committing directly to the trunk could result in blocking other developers. In such a case, you can create a branch in Subversion, set up a job on Hudson on that branch, and check this option.
You can then either:
Merge can be done repeatedly and bidirectionally (that is, you can merge between the trunk and the branch back and forth as many times as you need, so that the branch can follow the trunk closely)
This feature requires Subversion 1.5 on the server.