iASP works by running in response to a client connection or request (typically from a Web browser) routed through the Web server. When the request reaches the Web server, it is redirected to the iASP engine. iASP processes the application pages and executes embedded scripts, computes the results, and returns the data, packaged in HTML, to the Web server. The Web server then transmits the HTML back to the client's browser.
More information is available here.