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.

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.