Per Bill Koester comment on http://wiki.jenkins-ci.org/display/JENKINS/Join+Plugin
For a parallel job (one that is a child of the root job) it would be helpful if that job looked to see who triggered it. If the root/upstream project triggered it then the join plugin would continue as normal. If anything else triggered the build (ie. scm or manual) then it would build the downstream job after successful completion. As suggested a simple checkbox, that it activated within the child job, could turn on/off this behavior.