Flat vs. rounded back deadlift.
Here are my thoughts after trying both:
Round back: My shoulders are dropped to shorten the ROM. I make my arms long and reach for the bar. This sets my lats down towards my front pockets, keeping my lats/upper back tight.
My lower back starts in a neutral position, while my upper back is slightly rounded.
With “longer” arms, I’m able to start with my hips closer to the bar. This improves the leverage and allows me to have a more aggressive push off the floor.