git and hudson/jenkins - how to do incremental builds only?

How would I configure hudson/jenkins to checkout only the files/submodules,etc.. that have changed since the last nightly build? In our build system(ant/ivy), if it is checked out it gets built. So, I need to only checkout what has changed开发者_StackOverflow中文版 in order to do an incremental build.

Thats the default behaviour with the git plugin unless you set the project to do a clean build. What have you set up for it NOT to work?