The article by Cross does not talk explicitly about sweet spot size but it is implied from the increase in diameter and use of new materials. It is also part of MTM teaching as Oscar showed in a diagram that the modern sweet spot is almost as large as the head. It may have been an example of printer's devil, but Oscar got it right with his tour experience, unlike some of his followers who have never played at a high level. The across part seems to be simply a path of the swing. On reverse forehands, the across would come over the same shoulder, in the windshield wiper forehands it may be be more pronounced in the vertical plane, in other forehands it may be part of the solid contact when hitting through the ball to impart power. The main thing to note is that the naked eye misses how much hitting through the ball is present in powerful forehands, as we end up focusing on where the racket finishes. It is not tangential contact as some seem to believe. I don't know which Oscar video you saw, but if it is the one where there is an abrupt pullback at contact with a loss of balance to the side, I agree that it is not right.