It's straight up mandarin and it looks like nobody has taken a crack at it yet, so here goes. A more rigorous explanation of some of the trickier parts may be needed from someone else, but this should cover most of it.
-right before contact, the rackets angle changes to become a down-the-line" using change in wrist (thereby changing the path of the ball (1:36)
-if you do not make this change in wrist configuration, the angle of the racket takes the ball in the usual cross-court forehand (1:48) in this scenario, the contact is synced together with power from both the waist and the forearm (1:54)
-(2:00) now let's take a look at the different paths and trajectories of the strokes/shots, you'll see that they are quite different
-(2:05) (side-by-side on screen) the strokes are similar, as you can see, but the difference comes in the moment of exerting power (the chinese expression "fa li" is more subtle, but this is the best I can come up with). In the "sliding stroke" ("hua ban", again the best I can come up with) there are changes in both the fingers and wrist. At the moment right before contact, the wrist bends slightly back (translators note: and as I can see in the image, the racket angle opens slightly).
-(2:26) the narrator thinks these 3D representations are pretty good.
-(2:36) obviously in reality, the changes at the hand do not (and probably cannot) happen as slowly as they do in the cartoon
-(2:53) while the waist turns as usual in both strokes, it might be thought that the motion of the arm and the waist in the sliding stroke are somewhat opposite.This is such as the wrist and forearm are exerting power on the down-the-line and the waist rotates as usual from right to left in cross-court direction.
-(3:11) but they're not really opposite (lol)
-(3:17) actually for the "sliding stroke" the direction of power of stroke is already directed towards the down-the-line (between the start of the stroke and through the moment of contact
-similarly for normal cross court forehands the entire direction of power is already cross-court
-the distinction comes from the combinations of these two concepts: direction of power from your 1) stroke 2) racket angle/hand
-(3:50ish) if you direct your stroke/body power down the line but your racket/hand angle is still cross court, it will be very difficult to make the ball actually go down the line
-(4:09) but the "sliding forehand" is not the same as a down the line forehand anyway, the sliding forehand has a certain element of surprise. The sudden angle change just before contact is much more deceptive than having your racket angle already set up as such.