The InGo Activator (formally Login) Widget provides an ability to forward on the query string which can be useful if you need parameters passed on through from one page to another. However, it's use can be misunderstood and misapplied, so here's some clarification on what it does, and what it does NOT do.
What it Does
The forwardQueryString option takes the query string parameters of the current page and appends them to whatever URL you designate as the URL that the Activator Widget should send registrants to. It works the same, no matter what social network the user chooses or whether they choose to use manual or social registration. The option takes the query string from the current page (look at the address bar), and sends it on to the next page. If, for example, your next page already includes a query string (e.g., data-ingo-manual="http://example.com?eventid=123") and the activator widget is loaded on the page "http://example.com?source=fb_add", then the activator widget will append the current page query string (source=fb_add) to the next url resulting in the next url, "http://example.com?eventid=123&source=fb_add".
What it Does NOT Do
The activator widget is nifty, and seemingly magical, but it doesn't do magic, unfortunately. It doesn't take query strings from other similar URLs on the same page and append them to the url desinated as the next url. It doesn't take the query string that you are putting into your data-ingo-manual setting and forward that anywhere (that's part of your URL, so there's nothing to forward). It doesn't forward to InGo anything about the query string.
ForwardQueryString simply takes everything after the ? and before any # of the address of the page where the activator widget is loaded (again, look at your address bar, if you don't see it there, it's not going to be forwarded, as there's nothing to forward) and sends that data on to whatever you've designated as your next url, as additional query string parameters for the next url.
Precocious, maybe. No magic though.