All tasks done by individuals on a Scrum Team must be chosen voluntarily. If one Team Member, in any way, tells another Team Member what task to work on, this breaks the principle of self-organization that is essential to creating a high-performance Scrum team. Team leads, project managers, functional managers and other people in roles of authority to assign tasks must give up that authority completely when it comes to the people on a Scrum team. This self-organizing behavior allows individual team members to consider their own talents, capacity, interest, motivation etc., when choosing a task. All of those inner conditions are not as well known by other people and so assigning tasks tends to be sub-optimal. When a Team Member considers those inner conditions about him or her self, and also takes into consideration the needs of the team, an optimal task choice can be made. If someone in a position of authority does assign tasks, it creates a habit of deferring to authority which quickly destroys any possibility of a high-performance team developing.
Every Scrum Team Member should be working on tasks in the Sprint Backlog. Generally speaking, this should be one task at a time with little or no work done on work that is not on the Sprint Backlog. The visibility of the Sprint Backlog is an important part of Transparency within the Scrum Team. As well, doing one task at a time helps with Focus, another of Scrum’s values. If team members follow this rule, then the work of the Sprint is done in a reliable way. When team members take on multiple tasks simultaneously or when they take long breaks to do non-Sprint Backlog work, then the team’s focus is substantially diminished and overall productivity suffers.