The source code

    /**
     * @class Ext.tree.AsyncTreeNode
     * @extends Ext.tree.TreeNode
     * @cfg {TreeLoader} loader A TreeLoader to be used by this node (defaults to the loader defined on the tree)
     * @constructor
     * @param {Object/String} attributes The attributes/config for the node or just a string with the text for the node
     */
     Ext.tree.AsyncTreeNode = function(config){
        this.loaded = config && config.loaded === true;
        this.loading = false;
        Ext.tree.AsyncTreeNode.superclass.constructor.apply(this, arguments);
        /**
        * @event beforeload
        * Fires before this node is loaded, return false to cancel
        * @param {Node} this This node
        */
        this.addEvents('beforeload', 'load');
        /**
        * @event load
        * Fires when this node is loaded
        * @param {Node} this This node
        */
        /**
         * The loader used by this node (defaults to using the tree's defined loader)
         * @type TreeLoader
         * @property loader
         */
    };
    Ext.extend(Ext.tree.AsyncTreeNode, Ext.tree.TreeNode, {
        expand : function(deep, anim, callback, scope){
            if(this.loading){ // if an async load is already running, waiting til it's done
                var timer;
                var f = function(){
                    if(!this.loading){ // done loading
                        clearInterval(timer);
                        this.expand(deep, anim, callback, scope);
                    }
                }.createDelegate(this);
                timer = setInterval(f, 200);
                return;
            }
            if(!this.loaded){
                if(this.fireEvent("beforeload", this) === false){
                    return;
                }
                this.loading = true;
                this.ui.beforeLoad(this);
                var loader = this.loader || this.attributes.loader || this.getOwnerTree().getLoader();
                if(loader){
                    loader.load(this, this.loadComplete.createDelegate(this, [deep, anim, callback, scope]), this);
                    return;
                }
            }
            Ext.tree.AsyncTreeNode.superclass.expand.call(this, deep, anim, callback, scope);
        },

        /**
         * Returns true if this node is currently loading
         * @return {Boolean}
         */
        isLoading : function(){
            return this.loading;
        },

        loadComplete : function(deep, anim, callback, scope){
            this.loading = false;
            this.loaded = true;
            this.ui.afterLoad(this);
            this.fireEvent("load", this);
            this.expand(deep, anim, callback, scope);
        },

        /**
         * Returns true if this node has been loaded
         * @return {Boolean}
         */
        isLoaded : function(){
            return this.loaded;
        },

        hasChildNodes : function(){
            if(!this.isLeaf() && !this.loaded){
                return true;
            }else{
                return Ext.tree.AsyncTreeNode.superclass.hasChildNodes.call(this);
            }
        },

        /**
         * Trigger a reload for this node
         * @param {Function} callback
         * @param {Object} scope (optional) The scope (<code>this</code> reference) in which the callback is executed. Defaults to this Node.
         */
        reload : function(callback, scope){
            this.collapse(false, false);
            while(this.firstChild){
                this.removeChild(this.firstChild).destroy();
            }
            this.childrenRendered = false;
            this.loaded = false;
            if(this.isHiddenRoot()){
                this.expanded = false;
            }
            this.expand(false, false, callback, scope);
        }
    });

    Ext.tree.TreePanel.nodeTypes.async = Ext.tree.AsyncTreeNode;