This is part of the MSIE support for managing behaviors. With this method, you can access the value of an expression that was previously defined with the setExpression() method or which is defined within a style rule.
The rules that are used to construct a style are comprised of multiple expressions. You can use this method to extract the value of an expression from a style item.
Given the rule might contain a line such as:
The getExpression() method can be applied to the style object containing the rule with that expression like this:
The value 200px would be returned.