skin/frontend/base/default/js/opcheckout.js:289

// billing
var Billing = Class.create();
Billing.prototype = {
    initialize: function(form, addressUrl, saveUrl){
        this.form = form;
        if ($(this.form)) {
            $(this.form).observe('submit', function(event){this.save();Event.stop(event);}.bind(this));
        }
        this.addressUrl = addressUrl;
        this.saveUrl = saveUrl;
        this.onAddressLoad = this.fillForm.bindAsEventListener(this);
        this.onSave = this.nextStep.bindAsEventListener(this); // Bind nextStep() to onSave() event
        this.onComplete = this.resetLoadWaiting.bindAsEventListener(this);
    },

skin/frontend/base/default/js/opcheckout.js:388

// billing
var Billing = Class.create();
Billing.prototype = {
    
    // Other methods omitted for brevity
    
    /**
     This method recieves the AJAX response on success.
     There are 3 options: error, redirect or html with shipping options.
     */
    nextStep: function(transport){
        var response = transport.responseJSON || transport.responseText.evalJSON(true) || {};
        ...
        checkout.setStepResponse(response);
        ...
    }
};

skin/frontend/base/default/js/opcheckout.js:261

var Checkout = Class.create();
Checkout.prototype = {

    // Other methods omitted for brevity
    
    setStepResponse: function(response){
        ...
        if (response.goto_section) {
            this.gotoSection(response.goto_section, true);
            return true;
        }
        ...
    }
};

skin/frontend/base/default/js/opcheckout.js:130

var Checkout = Class.create();
Checkout.prototype = {

    // Other methods omitted for brevity
    
    gotoSection: function (section, reloadProgressBlock) {

        if (reloadProgressBlock) {
            this.reloadProgressBlock(this.currentStep);
        }
        this.currentStep = section;
        var sectionElement = $('opc-' + section);
        sectionElement.addClassName('allow');
        this.accordion.openSection('opc-' + section);
        if(!reloadProgressBlock) {
            this.resetPreviousSteps();
        }
    },
};

    /**
     * Reload relevant checkout progress step
     * 
     * @param toStep string One of billing, shipping, shipping_method, payment
     */
    reloadProgressBlock: function(toStep) {
        this.reloadStep(toStep);
        if (this.syncBillingShipping) {
            this.syncBillingShipping = false;
            this.reloadStep('shipping');
        }
    }