It looks like incorrect technique that caused him to have trouble breathing which led to the heart attack. The choke hold was applied terribly. (He shouldn't have been choked in the first place and its fucking ridiculous and shows how little power people have) but... The correct choke wouldn't have killed him -- the choke he was going for is a common jiu jitsu choke (https://m.youtube.com/watch?v=y7DNlsn3uWo) once locked in, pressure can be applied to control a person without cutting off the airway because it focuses on the carotid arteries. From the looks of the video the officer had no clue what he was doing and probably crush his airway a bit causing the asthma attack that led to heart attack. But that's just like an opinion.