"arch ur back" is a terrible cue for anyone familiar w modern motor learning
Stop going on line.
No instructor, worth a grain of salt, is saying, "Like doing back hand springs." It's just a key phrase that allows the student to "lean back" a bit.
Start small then add as the student progresses. Like eating a meal, one bite at a time. Brick by brick.
Plus remember, all these "on line tennis queens" and I didn't coin this phrase, they
are not in your company correcting anything your doing immediately. And this is a huge issue and always has been.
The greatest advantage, is having someone, with a good deal of instructional technique, "fixing" or "changing" what the student is doing, which is incorrect, immediately.
Yea, continue for a full day or a few weeks period of time, doing something which is incorrect and a detriment, i.e., incorrect technique. See how long it takes to repair the issue with an average student. Some players do incorrect things their entire lives! Why?
Over the years, I've had tons of players come to me with many issues related to improper teaching techniques from
poor quality tennis instructors. Many played tennis for 10-20 years and more, and still aren't doing things correct.
Like a ping pong ball, they're bouncing around from one instructor to another. Spending money, putting in the time and getting nothing in return. Seen this a thousand times.
Everyone is an instructor today. Gee, did I say this early on.
JS