Trigger is a plugin that calls external scripts (on the server) at the end of each client run.
First, add Trigger to the plugins line in bcfg2.conf. Then do the following:
mkdir /var/lib/bcfg2/Trigger
echo "#!/bin/sh\necho $1\n" > /var/lib/bcfg2/Trigger/test.sh
chmod +x /var/lib/bcfg2/Trigger/test.sh
Triggers are run with a series of arguments.