I noticed the ``checked`` attribute on following pages:
http://www.oldelvet.org.uk/2011/06/getting-to-grips-with-jelly-and-stapler/
http://jenkins-ci.361315.n4.nabble.com/Issues-with-boolean-parameter-in-config-jelly-td3703052.html

But I actually wouldn't know, since I am quite new to Jenkins plugins :)