Elbows Back, Chest Forward
Chest-supported rowing is not a crutch or inferior to regular rows. It offers one distinct advantage over non-supported rows:
you have something to push into.
This may sound like a minor advantage, but it is very valuable to learn how to row correctly. First of all, a row is not a row. You can row to grow your lats and you can row to grow your traps. Some even row to grow their biceps, which is stupid.
But that's another story.
Plus it's Monday and I wanna keep things positive. Chest-supported rows allow you to pull your elbows back while pushing your chest into the pad. This avoids hyperextending the back and keeping the upper body in place. You're not avoiding resistance, you're welcoming it. If the elbows go back and the chest goes forward, there's no choice for your back muscles to do the work. Plus you can "reach" at the bottom, essentially resulting in your shoulder blades rolling forward and stretching the traps.
What back muscles you hit depends on your grip and intention. Wherever your elbow ends up is the part of your back that get's stimulated the most. In the video I choose a medium, neutral grip to target the mid traps. This grip also tends to feel the best on my elbows and allows me to pull with very little biceps involvement.