Note that this only applied after you’d been charged in court. It did not apply to usual police interrogations that take place before you’ve been charged.

If you want to see just how frustrating the unguided “voluntariness” approach was, just try to read Frankfurter’s opinion in Culombe v. Connecticut, in which he tries for pages and pages and pages to make sense of it and justify it, but in the end only proves what a mockery it had become. It’s kinda sad, because he’d believed in it for so long. A simple guiding principle along the lines of common-law entrapment doctrine would have sufficed (i.e., if the subject had originally not intended to confess, and the police threatened or cajoled or otherwise made him change his mind, then the confession was per se involuntary). But…