In our context, across motion is tangential horizontal component of the racquet velocity that coincides to the racquet string bed. This across motion creates bollís clockwise spin.
Letís again analyze Federer FH.
In frame 5, Federer creates maximum tangential component (maximum brushing motion), but small normal component (to the string bed) of the racquet velocity. In frame 12 the racquet velocity has zero tangential component, so Federer could hit pure flat FH and really is able to hit at least through one - two balls.
The hitting through the ball is what the #1 junior in the world also says, one who hits with Raonic and Cilic. But that doesn't count over here. If he had said "hit across the ball" after "stalking" and "finding it with the hand," he would have become a hero here.