In my experience, this usually occurs because they want to tell you about _their_ research, instead of just their field. It's easy to give an intelligible answ

In my experience, this usually occurs because they want to tell you about _their_ research, instead of just their field. It's easy to give an intelligible answer that could be equally applicable to any of about 100 PhD theses. But getting someone to explain their specific contribution in under 5 minutes is often going to be pretty difficult.