The use of detention for psychiatric treatment is widespread and sometimes necessary. International human rights law requires a legal framework to safeguard the rights to liberty and personal integrity by preventing arbitrary detention. However, research suggests that extra-legal factors may influence decisions to detain. This article presents observational and interview data to describe how decisions to detain are made in practice in one jurisdiction (England and Wales) where a tension between policy and practice has been described. The analysis shows that practitioners mould the law into 'practical criteria' that appear to form a set of operational criteria for identifying cases to which the principle of soft paternalism may be applied. Most practitioners also appear willing, albeit often reluctantly, to depart from their usual reliance on the principle of soft paternalism and authorise detention of people with the capacity to refuse treatment, in order to prevent serious harm. We propose a potential resolution for the tension between policy and practice: two separate legal frameworks to authorise detention, one with a suitable test of capacity, used to enact soft paternalism, and the other to provide legal justification for detention for psychiatric treatment of the small number of people who retain decision-making capacity but nonetheless choose to place others at risk by refusing treatment. This separation of detention powers into two systems, according to the principle that justifies the use of detention would be intellectually coherent, consistent with human rights instruments and, being consistent with the apparent moral sentiments of practitioners, less prone to idiosyncratic interpretations in practice.