underscore.js的bind函数

先贴代码：

var executeBound = function(sourceFunc, boundFunc, context, callingContext, args) {
if (!(callingContext instanceof boundFunc))
return sourceFunc.apply(context, args);
var self = baseCreate(sourceFunc.prototype);
var result = sourceFunc.apply(self, args);
if (_.isObject(result)) 
return result;
return self;
};
// Create a function bound to a given object (assigning this, and arguments,
// optionally). Delegates to ECMAScript 5's native Function.bind if
// available.
.bind = function(func, context) {
if (nativeBind && func.bind === nativeBind) return nativeBind.apply(func, slice.call(arguments, 1));
if (!.isFunction(func)) throw new TypeError('Bind must be called on a function');
var args = slice.call(arguments, 2);
var bound = function() {
return executeBound(func, bound, context, this, args.concat(slice.call(arguments)));
};
return bound;
};

想问下，executeBound(func, bound, context, this, args.concat(slice.call(arguments)))这个里面的this是否就是指代调用_.bound的对象，然后后面为何要做(!(callingContext instanceof boundFunc))这个判断，网上查了好久，没查出来，请高手指导下，非常感谢