You can read the basics of IE behaviors in Microsoft's documentation:
http://msdn.microsoft.com/en-us/library ... 85%29.aspxIn a nutshell, think of the behavior property just like any other CSS property: any element that matches the selector will get that property as part of its overall set of styles. But unlike most style properties which simply modify a single aspect of the element's presentation (e.g. its background-color), the behavior style actually downloads and runs a bit of JavaScript (the .htc file) in the scope of that element.
In PIE's case, the JavaScript in PIE.htc is executed for each element to which the behavior is applied. This script examines the set of CSS3 style properties applied to the element, parses them, and creates a graphical representation of those styles using VML.
I'm not sure what you mean by "attaching it to one global element" -- it does have to be attached to each element that you want PIE to render. You can often attach it to multiple elements at once using a single selector of course.