RDL Movement

For an RDL(romanian deadlift) movement, i saw a gif of a guy doing with without his back arched inward. Is this actually the proper way to do it? I know ive seen other movies/gifs with the back arched inward…like squats and the such.

What major difference would this impact anwyays–besides the potential injury?

You want your chest high and your ass out, basically the same posture as the deadlift/clean/etc. Rounding out your back usually comes from using too much weight and can cause injury to the lower back (hence the age-old saying “lift with your legs, not with your back”).