How does the sight of Fortinbras and his troops change attitude?

When Hamlet sees Fortinbras and his army, he is struck by their determination and sense of purpose. He notices that they are willing to risk their lives for a cause they believe in, and this inspires him to take action. He realizes that he has been procrastinating and that he needs to take action to avenge his father's death. He also admires the idea that Fortinbras was able to take action without hesitation whereas Hamlet found it difficult to do the same.