Web Forms :: When Is Application_BeginRequest Called?

I have two versions of a website. One runs on IIS server on a local PC, the other runs on the ASP.NET development environment included on Visual Studio (localhost). Apart from that, the file stucture of both is the same, although only the one running on IIS works properly, and this seems to be related to the fact that the IIS version calls Application_BeginRequest() where the other doesn't.

I've looked at the differences between IIS server and the ASP development environment in: [URL], and it says that IIS and the ASP development server deal with static content in that for IIS in that on IIS static content does not go through the ASP.NET runtime like it does on the dev environment (but I'm not sure exactly what this means).

I've done a number of tests involving images, etc, and have noticed that when the image is of the form:

<img src="Image/MyImage.jpg...>

the IIS server version calls Application_BeginRequest(), but the visual studio development environment version doesn't.

However, if I change the above code to use:

<img src="<%=ResolveUrl("~/Image/MyImage.jpg")%>" ...>

it will call Application_BeginRequest().

In my case, it is necessary to call Application_BeginRequest because this creates a new path and calls
RewritePath().

The problem occurs when using links such as <a href="..." ...>, because if I use static content it won't call Application_BeginRequest() and the path won't be re-written. But If I change it to href=" %=ResolveUrl("~/...")%>" (for some reason, it will only call BeginRequest() if the path begins with a tilde ~), the path is re-written incorrectly.

I'm not really sure how to find out why the IIS version is calling Application_BeginRequest() from all the time (the callstack just says "external code"), or why it is calling it whereas the local dev server version isn't.

If anybody can explain this, or knows of any sites that go into this so I can master the basics, I'd be very grateful.