I will preface my opinion by saying I *think* I know what Trevon Diggs was trying to do. I think he was attempting to dislodge the football from George Kittle's grasp by slamming his shoulder into the ball. It is a shame because he would have timed it well WITHOUT launching into Kittle's body. Unfortunately, what Diggs actually did would have likely drawn a flag if he had made contact. Here are the relevant portions of the rule:
https://operations.nfl.com/media/5kvgzyss/2022-nfl-rulebook-final.pdf (pages 58-59)
Rule 12, Section 2, Personal Fouls, Article 9, PLAYERS IN A DEFENSELESS POSTURE. It is a foul if a player initiates unnecessary contact against a player who is in a defenseless posture.
(a) Players in a defenseless posture are:
(2) A receiver running a pass route when the defender approaches
from the side or behind...
(3) A player attempting to catch a pass who has not had time to clearly become a runner...
(b)
Prohibited contact against a player who is in a defenseless posture is:
(1) forcibly hitting the defenseless player’s head or neck area with the helmet, facemask, forearm, or shoulder, even if the initial contact is lower than the player’s neck, and regardless of whether the defensive player also uses his arms to tackle the defenseless player by encircling or grasping him;
(2)
lowering the head and making forcible contact with any part of the helmet against any part of the defenseless player’s body; or
(3) illegally launching into a defenseless opponent. It is an illegal launch if a player (i) leaves both feet prior to contact to spring forward and upward into his opponent, and (ii) uses any part of his helmet to initiate forcible contact against any part of his opponent’s body. (This does not apply to contact against a runner, unless the runner is still considered to be a defenseless player, as defined in Article 9).
Notes:
(1) The provisions of (b) do not prohibit incidental contact by the mask or helmet in the course of a conventional tackle or block on an opponent.
(2) A player who initiates contact against a defenseless opponent is responsible for avoiding an illegal act. This includes illegal contact that may occur during the process of attempting to dislodge the ball from an opponent. A standard of strict liability applies for any contact against a defenseless opponent, even if the opponent is an airborne player who is returning to the ground or whose body position is otherwise in motion, and irrespective of any acts by the defenseless opponent, such as ducking his head or curling up his body in anticipation of contact
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Diggs lowered his head during his attempt. That would have likely drawn a flag if his hit had landed. Instead of a hitstick approach to knocking the ball loose, I believe form tackling would have possibly done what Diggs wanted to do and not drawn a flag.
Form tackling forces the defender's eyes up, which keeps his helmet up. My old football coach (rest his soul) called it 'seeing what you are hitting'. The defender wraps up the ball carrier, keeping his head in the carrier's body and NOT driving the helmet into the head or neck area.
This is what Diggs could have done instead of trying to finesse a big hit on the ball with only his shoulder. The force behind a form tackle, along with putting helmet on the ball, might not have dislodged the ball from Kittle, but it could have. It is a fundamental of basic football players, taught in junior high up, that professional players, in particular, either shy away from because it is not flashy enough and does not deliver the intended force. Or it is a fundamental that is simply forgotten how to perform because it is not practiced routinely.
A proper hard hit form tackle brings the 'Ooos and Ahhhs You Got Jacked Up' from fans. Unfortunately, professional football players do not employ it nearly enough. Diggs certainly didn't.