Interventions against excessive smartphone use have predominantly focused on browser extensions and nudges within a problematic digital environment. Smartphone apps are applicable to different environments and more transparent than common interventions. one sec is a self-nudging app that helps users to reduce their consumption of selected apps. We show that one sec nudges users in one out of three cases (36%) to dismiss their consumption attempt after having opened a target app. Users also attempt to open their target apps less often (37%) in general. A controlled experiment disentangles one sec’s psychological mechanism. Displaying users with an option to dismiss their consumption attempt is one sec's most effective feature, while the friction it produces by time delay also plays a role.