Can somebody PLEASE explain what is the difference? Wasn't one supposed to keep his backed arched or neutral when lifting???
They can be done both ways. Most prefer or recommend the arch-backed good morning, but sometimes, the rounded back version is good too. Along with seated versions.
In a good morning, most try to keep an arch but when heavy weights are used, the back rounds at the bottom.
An arch-back GM is a different exercise, similar to a romanian dl. Set up with a wide stance, arch your back hard, and push your abs out. Now push your butt back. Your range of motion will only be around 8 inches, and you mustn't lose the arch. Keep your chest up and arch up instead of firing through with your hips.