(Copied from Meta-Wiki.)
This template consists of an if-construct without then- or else-part, where the evaluation of the condition involves the evaluation of the parameter 1. Thus it does not return any result, but it forces the evaluation of the parameter.
A call of this template with the call of another template as parameter has a functionality that is similar to that of a hidden category. Regardless of existence and content of the template, nothing is returned, but nevertheless the calling page is listed in Whatlinkshere applied to the target page, even if that does not exist. Also, in a preview of the calling page the target template is listed in the "Templates used" list.
This template can for example be used for monitoring parameter usage.