I don't know what hold the officer is trying to make. I think he initially tried some sort of triangle arm choke, realised it would never work applied to someone that much bigger, and then held on however he could because he already had hands on Garner. Reports of it being an attempted takedown make even less sense, given the size difference (football players try to tackle at the waist for a reason).
The officer hangs on, how is blocked by other people on the video I saw. By the time he has Garner on the ground, his arm is around Garner's neck with the elbow up near a shoulder. This is a bad position, assuming he's trying for a blood choke, as it leaves the pressure on the trachea instead of the carotids.
So, regardless of intentions, the officer fucked up, and likely didn't know how much damage he'd done.
So the first bit of info I need and don't have: what related holds are SoP by NYPD policy, and did he at least attempt to use them, and correctly?
Second bit of info I need and don't have: what's SoP for administering first aid to suspects in custody? Seven minutes waiting around for ems happened for a reason.
I think the ultimate reason for the Grand jury's decision lies somewhere with those bits of info.
Fucking tragic any way round.