What is considered to be an involuntary confession in a criminal case?

A confession is deemed involuntary and hence inadmissible if procured by an express or implied promise of benefit beyond that naturally flowing from a truthful statement (People v. Johnson, 70 Cal.2d 469, 479, 74 Cal.Rptr. 889, 450 P.2d 265), or by an express or implied threat that the failure to make a statement will result in consequences adverse to the suspect. (People v. Brommel, 56 Cal.2d 629, 633--634, 15 Cal.Rptr. 909, 364 P.2d 845.) Conversely, a confession is deemed voluntary and hence admissible if obtained in response

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