Emily Longley could have been rendered unconscious in seconds if she was strangled in the "sleeper hold", a British court has been told.
Pathologist Dr John Payne-James said such a hold could have rendered her unconscious extremely quickly and could result in death.
Emily, a New Zealand student, was allegedly strangled by boyfriend Elliot Turner, 20, in a jealous rage at his parents' home in Bournemouth, Dorset.
Turner has been charged with her murder and with trying to pervert the course of justice, and is on trial in the Winchester Crown Court.
A jury has been told the wealthy jeweller's son was jealous and possessive, and attacked Emily, 17, on May 7 last year during a drunken argument.