Swarming is the process of large number of team members working on the same task to complete it. In great self organizing teams, swarming is an automatic process, when there is a crisis / delay. In not so great teams, the project manager / scrum master has to allocate resources to work on a particular task, which is becoming a show stopper or it prevents in achieving certain urgent milestones.
Technically a sprint is termed as a failed sprint, if the planned stories are not completed. A faulty component can prevent all other dependent components to fail. In such scenarios, team swarms and get the problem solved.
It is more like the all out game played in the ‘total foot ball’ by the lagging team, during the closing minutes of the match.