Update field via name?

Hello,

I'm updating a form field with a value from another field. It works with

$x = $_POST["input_11"];
$_POST["input_42"] = $x;

But someone mentioned in this thread:

http://www.gravityhelp.com/forums/topic/combining-fields

"With the new version of Gravity Forms you can now name fields which makes this hack much easier"

Is this documented somewhere?

I can't even get this to work:

$_POST["optionlist"] = "test";

I've made the field label, admin label & parameter name "optionlist"...

Thank you for any help!

Thanks!