The tilt is legal until that non pivot foot is in front of that imaginary line. It's been a POE for a few years now, but it's a PITA to deal with. I had one kid legal for about 5 or so innings, but each inning his sideways tilt would move up about an inch or so. So come the 5th or 6th inning when you now recognize he's illegal - you are supposed to call the Illegal Pitch. Some coach is going to ask and note that he's been standing that way the whole game and why are you "picking on it" now that we're in the later innings. (The old adage being don't call late what you won't call early). So your best defense to that is as PU kick the catcher in the butt and tell him to talk to his pitcher or as a FU talk to the 1B-man and tell him to talk to his pitcher. Of course in both cases you'll have the other coach wondering why you're giving an advantage to the other team. Also that assumes the C or 1B-man actually knows what you mean and has a chance at explaining it to the P. It's truly a no win situation for all involved especially with no one on base. The whole problem is made harder because perhaps a small percentage can actually explain what the right foot position is. Another much larger percentage would prefer to ignore it leaving the smaller percentage calling it and taking the grief.
Personally here's what I wish would happen... The only "legal" set or stretch position is one where the *entire* pivot foot is in contact with the rubber. Every other crazy position some coach dreams up thinking he'll get some advantage is considered wind-up position. The only time it's legal to jump turn to 1B is when you're in set. Also depending upon the condition of the hole at the pitcher's plate, I wouldn't get "too technical" over how much of the foot is in contact (2019's picture). Sure toe or heel only in contact, he's gotta fix it, but avoiding the hole and having 2-4" of the heel or toe off the edge - less of a big deal even though technically not legal. Sure, we could ask to have the hole filled in as soon as we find the grounds crew... They could put some clay in there, tamp it down, delay the game by 20 minutes... Great. joy... I think this just happens to be one of those cases where a written rule doesn't quite make sense under game conditions. Makes me wonder how many rule writers actually work games on less than perfect fields or games in general.
Did you also realize that when in stretch that the pitching hand "shall be down at his side or behind his back"? Try explaining that to a coach or pitcher. But I have seen a RHP with the ball on his front hip be able to make one heckova snap throw to 1B while he's jump turning, so I get why it's a rule!