The cue to pull across (and to a certain extent to also actively pull back at impact) works. But does it actually go across only? No of course not. As the swing is circular the swing can be manipulated with a lot of nuance. If you see how djokovic hits when he finishes his fh over his shoulder there seems to be active use of his bicep to pull back the stroke as well as the across aspect. Does the racket pull back and slow down? No. The racket seems to accelerate faster in the arc across. One advantage I've found in focusing on the across aspect of a fh stroke is your shoulders are always centered over your hips. I've tried numerous cues when coaching my 11 year old son and focusing on the across aspect of the stroke and with an emphasis on starting the swing slower then accelerating through impact really helps with pace and spin. One thing I add that Oscar does not really touch on much is the use of the legs. I found that by using the knee bend and lifting into the stroke it adds more racket head speed with out the need to swing any faster.
I am still not certain the active pull back advice by Oscar is right for every occasion. As I taught my son a straight arm forehand when he hits with a normal wiper motion he hits harder than when he actively pulls back. The finish is different as well as spin. My son gets more top spin if he uses the cue to pull back. But the ball is heavier with a normal wiper motion using the straight arm. And the ball has enough clearance over the net regardless. And on wide balls on the run to the fh he is hitting a reverse fh. Perhaps the active pull back cue can be used more effectively to affect the wide fhs on the run? To add more height and spin without resorting to a reverse fh?
Regardless my son prefers to not use the pull back cue as he hits well with a normal wiper straight arm fh. The ball is moving fast and with a lot of spin. And I'm not about to change things at this point after 6 years of working on his fh.