It used to be a key (ctrl) but now (v4) there is a small "D" button on the dictionary and commentary window toolbars that does the same. The button is selected (in) by default which causes all click actions to go to the default resource. If the button is not "On," the actions will be performed on the current resource.
Developer, Bible Analyzer
But to him that worketh not, but believeth on him that justifieth the ungodly, his faith is counted for righteousness. (Rom 4:5 AV)