A former military combat instructor says a fellow Marine veteran misused a combat maneuver when he fatally choked a homeless man aboard a Manhattan subway last year.

The military trainer said that Marines are taught to render a person unconscious in seconds. Mr. Penny held Jordan Neely, a homeless man, for six minutes.

Penny held Neely in a chokehold for about six minutes — including for more than 50 seconds after Neely stopped moving — according to evidence presented in his criminal trial.

A subway commuter who helped an ex-Marine restrain an agitated man aboard a Manhattan subway last year testified that he tried to convince the veteran to loosen his grip around the man’s neck.

Done right, the maneuver should knock a person out without killing them, according to a Marines combat instructor who trained Penny. But held too long, the technique can restrict the flow of blood

When Daniel Penny fatally choked a homeless man aboard a Manhattan subway last year, the veteran appeared to be using a combat technique that he learned in the Marines, according to a martial arts instructor.