Only comment I have is to keep the chin tucked in,
meaning don't crane your neck up to stare straight ahead.
It's a great unilateral movement to hit the posterior chain while improving balance/stability and coordination. I like to incorporate it since it's one of the few single-leg exercises recruiting the hamstrings with little focus on the quadriceps.
She makes it look deceptively easy, but that's far from the truth. First, poor hip mobility will slap any attempt you make. Second, it still involves a somewhat unique awareness of how you're moving [proprioceptive ability] compared to other exercises, say like the lunge or step-up.
Through a little trial and error, I've managed to smooth out some kinks making it easier to perform.
1) Arch your back hard. This is often reserved for the major compound lifts like deadlift, squat, barbell rows, and benching, but this also prevents the torso from flexing at the low back as well as keeping it rigid. Chest pushed out, shoulder blades pulled down, and stick your butt back.There a few ways to program the single-leg Romanian deadlift into your own training. Weightless is great to add in your warm-up. Since this isn't an exercise where you go heavy, I recommend in lifting sessions to stick with light to moderate weights for sets with reps being 5-6 or higher.
2) The arched back sets us up for initiating and executing the movement, which in my opinion is the trickiest part. The cue I developed was to visualize it as closing a ship's hatch. Take a look:
In the first image, the hatch is only able to move at one fixed point to be closed - indicated by the blue arrow. Likewise, in the next image you should imagine the same thing. Only move at the fixed point of the hip, again the blue arrow, and think of "closing the hatch," almost as if you were trying to make the top of your hip/pelvic bone touch the very top part of your thigh. On the way back up "open" the hatch.
Often it's thought of as the entire torso moving. However, that acts as a distraction. With your back arched, you won't have to worry about rounding out and can focus on the hip pivoting.
3) Weight distribution can act as a hindrance. Use different weights in each hand if your body is falling towards one side or the other. For example, if doing reps on the left foot and you find yourself falling to the right then instead of using two 30lb. dumbbells use a 35lb. in your left hand as a counterweight and a 25lb. in the right.
4) The last key factor is your footing and how you reach down. Keeping your foot firmly planted will prevent swaying. As for your hands when you descend, reaching over midfoot or over the knuckles of your toes is a good spot. Essentially, let your hands hang but don't let them deviate so far away they make you fall.
Though make no mistake, this is an exercise that takes practice. If you're new to it then practice with your bodyweight, use a wall to assist yourself, do partial reps, or try a regular bodyweight Romanian deadlift on both feet and slowly switch to a staggered/split stance (one foot placed further back than the other).
Now then, go work those hammies!