在英文雅虎上搜到如下内容，基本解决自己的问题，这可是msdn上都没有的。
Late Binding
Late binding is performed if the type of variable that holds the object instance is defined as Object or Variant: Dim objOutlook as Object or Dim objOutlook as Variant.

The VB run-time library has a set of functions for late binding calls. Their names are constructed using 'LateMem' with various prefixes and postfixes: __vba[Var]LateMem[Named][Call][St|Ld][Ad|Rf].

For example:
__vbaLateMemSt
__vbaLateMemCallLd
__vbaLateMemNamedStAd

We will call these 'LateMem functions'. Each of them receives the name of the calling method (as a string), the number of the method's parameters, the parameters themselvers, and (optionally) a pointer for the result value. For those who are familiar with COM technology basics, we can say that all LateMem functions use IDispatch interface.