I am trying to pass two simple parms with the passthrough query string feature.  But there is a mysterious text field on the far right that must be filled in.  No documentation or forum comments I can find say what to put in this field.  If you leave it blank you get the following error:

"Invalid Parameter The specified parameter options are invalid. Ensure the Name and, if required, type argument are both specified."

I have tried a bunch of things, including "string" "code" I even tried "gumby" (just to prove it will take anything). But none of these will make the passthrough querystring work.

What is the secret ?

Yes, 
I am using the latest version of everything (DNN 4.8).

However, I answered my own question last night.

I realize now that the textbox is to contain the source querystring item name.   The error message fooled me into thinking that it was for the object "type".   But now I realize that, instead of simply passing the query string element through to the target page, the iframe is able to convert the name.  So that if the URL on the DNN page contains &pigname=porky  but the target page is looking for &pigtitle=porky, you would put "pigname" in the box at the right ( and pigtitle on the left). 

 Would be nice if the textbox had a label though.

You are correct. And, yes, It will be labeled.

HOWEVER, your quesion brought up an interesting issue: HumanFriendly URLs were introduced into DNN and initially created a small issue - QueryString parameters were often mishandled. This affected not just IFrame, but many more modules that rely on QueryString parameters for passing vital data.

This issue has been cured in DNN 4.8 So if you are running earlier versions of DNN and can't upgrade, make sure you are not using Human Friendly URLs (the setting is on Site Settings screen)  and  you are using  the core UrlRewrite dll.