Having seen it now - it looks worse than I'd expected from having read forum posts. Red is in the harsh-but-fair category for me.
He transfers the ball to his left arm, freeing up his right, which then comes up before impact, with the elbow bent to 90*.
The tackler then essentially head-buts Fa's elbow region and KOs himself.
It's not deliberate, it's instinctive - and perfectly believable that Fa didn't even realise he'd raised his arm; but it's reckless.
IMO he simply mis-timed a push off against the tackler, and moved his arm out too early; but it results in a strike to the head, with the elbow.
Had he left his elbow tucked in against his ribs - basically, timing it correctly; then the outcome would probably have been the same; but it would have just been a rugby incident, no foul. Had they not had any replays, it would probably have been a red card; as 1 look in real-time looked bang to rights.