Normally you run a promotion by hitting the promotion button, but sometimes a script to run it programmatically will be handy, for instance when you want to run the promotion as a step as part of other process:
import com.pmease.quickbuild.ScriptEngine;
def srcConf = system.getConfiguration("path/to/src/configuration");
def buildToPromote = srcConf.latestSuccessfulBuild;
def promotion = srcConf.findPromotion("your promotion name");
promotion = ScriptEngine.instance.installInterpolator(promotion);
def vars = ["name1": "value1", "name2": "value2"];
promotion.promote(buildToPromote, vars);