This dance has a lot of moments to show off musicality and poise. Basically relying on the girls to hit every beat, not look awkward with travelling, and strike nice poses. I do feel that, as this is a long time ago, dance practices didn’t have the same emphasis on being polished but rather looking like stripped down behind the scenes moments. So, I believe some members are just being more laid back/casual than you might see now. I would say no one is going full out for the entire dance here.

There’s quite a few mistakes or awkward moves and yet they’re quite in sync, more so than I expected considering how casual they’re being.

Here is how long I focused on each member. Remember, this is not telling of their overall skill or even their overall performance in the practice as I couldn’t give 2nd or 3rd place points which would change this up a bit.
Hyoyeon = 49s
Yuri = 45s
Yoona = 37s
Seohyun = 34s
Sooyoung = 18s
Sunny = 18s
Jessica = 9s
Taeyeon = 8s
Tiffany = 2s

Here is what I noticed about each member:

Hyoyeon
Brilliant at moving her whole body in a smooth and accurate way. She adds the extra flair to each move in a subtle way and knows when to be sustained vs sharp etc. It’s virtually perfect for what it needs to be in this dance. I would say she sometimes makes minor mistakes or could be a bit more graceful on some moves but that’s about it.

Yuri
She’s got a great mix of strength and technique. She is knowledgeable about how to move nicely, keep balance, hold her body, and add that extra bit. She is the least likely to look awkward while waiting for the next move. When travelling, she struggles to be as smooth as when she is still. She also overdoes it sometimes when subtly should be favoured.
Yoona
She probably is the most full out throughout the dance. Yoona is great with her upper body control and has a good sense of timing. Her strength is in smooth movement but she has some awkwardness with her limbs. What she adds to the moves is not as subtle as Yuri or Hyoyeon either, even when it probably should be.

Seohyun
She catches little details and is graceful with her wrist and hip movements. She also has a good sense of timing. I think perhaps she lacks confidence at this point as she seems to be half-heartedly doing the sassy parts. She also just lacks the level of control and flow that the dance line has.

Sooyoung
I think she’s purposefully underdancing here since it’s just a practice. Usually she is a lot better. That or maybe she just wasn’t feeling her best which is fine too. Notably, she’s doing a lot less with her upper body that she is her lower. However, Sooyoung often does shine with her hip and footwork more than upper body stuff so maybe that’s just natural for her so when she’s taking it easy, the gap is more apparent.

Sunny
Opposite of Seohyun. She’s got the sass and really throws herself into moves. However, this sometimes costs her the accuracy. She moves in and out of being very good and being off in either time or position throughout the dance.

Taeyeon
I believe Taeyeon is underdancing slightly just based on her performance compared to live stages. Still, she gets the hits and is good with the moves, rarely actually doing something wrong. She could be adding that little extra though, seeming a bit lackluster in the times when she could steal the show.

Jessica
Another member I think is under dancing for this practice. Again, it’s not a bad thing as the expectation was that it’s a casual setting. She does have great control over her body and when she gets it, she’s one of the best, slowing easily from shoulder to hip or vise versa. However, she often makes mistakes, finding her limbs in the wrong position. She could also take a lot more moments to add a little extra (a head nod, really pushing out the hip, a sharper shoulder move etc)

Tiffany
She seems very unsure what to do for most of this dance and often doesn’t flow well from move to move. It may just be how she was feeling this day as she seems to cut quite a few moves or hesitate on things. She is good at shoulder and hip moves but seems nervous about them. Fany is by no means a bad dancer, but this particular video does not do her any favours.