Need to keep the arms in the natural position at the side of the body if the arms are away from the side it is deemed an unnatural position it doesn't matter whether you have your back to the play or not. Having said that we see players using their hands and arms unlawfully every week without penalty.
But Uefa said the distance the ball covered was "not short" and Kimpembe's arm was "not close" to his body. Based on this, it added that "the impact could therefore not be unexpected" and "the defender's body [was made] bigger thus resulting in the ball being stopped from travelling in the direction of the goal."The referee, therefore, awarded a penalty kick."Given that the referee did not recognise the incident clearly during live play [referred to as serious missed incident in the VAR protocol] an on-field review was conducted."
UEFA have explained why it was correct to award the penalty:QuoteBut Uefa said the distance the ball covered was "not short" and Kimpembe's arm was "not close" to his body. Based on this, it added that "the impact could therefore not be unexpected" and "the defender's body [was made] bigger thus resulting in the ball being stopped from travelling in the direction of the goal."The referee, therefore, awarded a penalty kick."Given that the referee did not recognise the incident clearly during live play [referred to as serious missed incident in the VAR protocol] an on-field review was conducted."https://www.bbc.co.uk/sport/football/47495707
Another change to the laws of the game means that if the player's arms extend beyond a "natural silhouette", handball will be given, even if it is perceived as accidental.