I have come around to seeing why this drill, in concept, might be the one of the ultimate ways to correct fundamental misunderstandings about a backhand swing. It is also one of the ultimate ways to obtain fundamental misunderstandings about the swing, though.
Starting at the right pec seems like the biggest hurdle. I understand the feeling, and even still, without some mobility in the arm it can be difficult to capture the hit from this position if you don't realize the other movement he is doing.
If this drill is completely opaque to you still, Im curious if taking 'start from the hit' even MORE literally might help solidify the goal.
Try standing in a slightly closed stance, with your arm held out straight at about 11 o'clock to the target, where you ideally release an actual throw. You will have to keep the arm rigid for this part, but release the wrist. Use just your lower body to shift weight until you can make your wrist rock back and forth rhythmically, making you look like some kind of deranged jazz fan snapping to a beat.
Slowly start to release your forearm, but continue to artificially stop everything at the 11 o'clock. Just play with this, and do it gently, the goal is absolutely NOT power, it is purely to feel how you can power the arm through these positions with weight shifting as the primary engine. It won't be pretty, and its not the goal to be pretty. The goal is only to find rhythm that starts to cause the shapes that are involved in the swing. I find that doing this seems like I start to make gradually larger figure 8's. This is the hidden magic of why the drill looks easy when Beato does it, and why it seems useless when lots of us do it for the first time.
Keep playing with this until the figure 8 is big enough to get your hand near the right pec ( this is not an absolute position, go with what your body wants to do somewhat ). Keep sending the arm back out to the 11 o'clock GENTLY. I don't want anyone to hyperextend an elbow trying this, I seriously mean to do this at a speed/power that would throw a disc 15 feet. Sending the arm to this 11 o'clock position is going to remove the tendency to 'spin out', and force you to 'shift from behind'. If you don't start shifting from behind, you will likely find yourself sending your arm to 12 o'clock or worse.
Play with your front leg, and feel how extending it juices up sending your arm to the 11 o'clock position. Again, slowly, but try to feel this, don't rush through and try to throw anything, just play with the feeling.
When you think you actually can feel what I am talking about, do a few of these slowly, then ALSO SLOWLY, but with a little bit more power, do the same motion and allow your arm to follow through completely. Don't stop it at 11 o'clock, but keep that position as the target of your momentum with the arm.
This might be too cryptic to help and might need a video or something, I just feel like once you 'get' the hit, so many things fix themselves. The timing becomes a natural feeling movement, instead of a series of poses that you are trying to manipulate consciously.