This is a dumb dumb rule. They’re basically making it illegal if an offense can jump as soon as the defense moves or stems the front.
Not sure what would even initiate making this a discussion.
As I have been informed (many times) it is called “stemming.”
The act of stemming is not illegal - defenses can still do it. But if it results in the offensive line flinching, false starting, etc, then it is a delay of game penalty on the defense