The source code

    /**
     * @class Ext.layout.AutoLayout
     * <p>The AutoLayout is the default layout manager delegated by {@link Ext.Container} to
     * render any child Components when no <tt>{@link Ext.Container#layout layout}</tt> is configured into
     * a {@link Ext.Container Container}.</tt>.  AutoLayout provides only a passthrough of any layout calls
     * to any child containers.</p>
     */
    Ext.layout.AutoLayout = Ext.extend(Ext.layout.ContainerLayout, {
        type: 'auto',

        monitorResize: true,

        onLayout : function(ct, target){
            Ext.layout.AutoLayout.superclass.onLayout.call(this, ct, target);
            var cs = this.getRenderedItems(ct), len = cs.length, i, c;
            for(i = 0; i < len; i++){
                c = cs[i];
                if (c.doLayout){
                    // Shallow layout children
                    c.doLayout(true);
                }
            }
        }
    });

    Ext.Container.LAYOUTS['auto'] = Ext.layout.AutoLayout;