A kick up the backside or arm round the shoulder. Delivered at the right time, in the right way both are recognised managerial responses. Some people respond better to one than the other. Managers may be predisposed to confrontational management or supportive management. Supportive management is presented as a good thing, although staff often complain there isn't much of it. Supporting and encouraging can be more effective than criticising and fault finding but if an individual is not living up to their potential, is failing to make the most of their skills because they are distracted, unfocused and coasting then a kick up the backside may be appropriate.
Confrontational management is often viewed negatively, characterised by a lot of shouting and accusations. But confrontational leadership is more about honest feedback, being explicit about what needs to change and how to do it. It's about not being afraid to confront the individual, something that many managers shy away from preferring a quiet life and not wanting to open themselves up to a complaint of bullying. It doesn't involve shouting.
Just as supporting and encouraging is not about being soft, confrontational leadership is not about being aggressive. The new guy doesn't know how we do things but with support and encouragement will quickly become a valued member of the team. An experienced individual has developed some bad habits and is living off past successes, for the good of the team this cannot go unchallenged, the manager needs to confront the individual with specific examples of poor performance, inappropriate behaviour, and make clear what is expected and what will be the consequences if these aren't addressed.