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There is head contact with every second tackle, that's a lot different to head first contact and that didn't happen.
My viewing of the footage is that he hit flat on his back rather than head, or even shoulders, contacting first.
You don't have to have made head first contact to get knocked out.
As I said originally for my point of view, no doubt a Yellow for an ugly tackle, but anything further is pathetic, regardless of the injury status of the tackled player.
What was happening to Bam in that ruck that went unnoticed live and under review had far greater potential for serious injury to the neck than the tackle did.