Could you please provide link to ISU rule or definition, which says that "jump begins when first foot leaves the ice"? I am genuinely curios

Does it mean that Boyang Jin's 4Lz is also underrotated? I mean, looking at the same Boyang's moment of "leaving the ice with first foot"
https://youtu.be/VziXOiM8Jaw?t=29 - Boyang's picking foot is facing almost in the same direction as Anna's picking foot - and they both landed with similar direction of the landing foot as well. If you meant Anna's
face and not her foot - it can't be right because all URs are always counted by
landing foot position (which is the
same foot as
picking foot)- judges are not interested in body position and never were. Only rotations of one and the same picking-landing foot matters when counting number of rotations. Therefore, if we try to apply your rule to Boyang
https://youtu.be/VziXOiM8Jaw?t=73 because they landed identically - it should mean that Boyang's jump is URed as well - because he
isn't facing in the same direction when he lands.