a) It was hardly a blast,
b) Pointing at the full back doesn't constitute the arm being in a natural position
c) It was not unavoidable, the arm being where it was puts it at risk
I agree that a penalty for that offence is incredibly disproportionate. I agree the law is ambiguous, but by what referees go by, deliberate means the arms not being down y the side essentially. When jumping the arms will flail naturally so there is leeway there but now with feet planted by the sides. The rationale is that your hand has influenced the play, and it's in a position it shouldn't be in. End of. The argument that does carry weight is it being ball to hand, but I don't think it was a ball hit super hard, I think it was clumsy from Sissoko and the fact it hit his chest first furthers the point that he had time to get out the way. Good that they are changing the rules I suppose but doubt they'll do a good job of it, nothing fouls like that should not lead to game changing penalties, unfortunately I don't think the ref really had a choice.