Okay, okay, it's not Tuesday but I didn't want to wait til next week.
When I do workshops and teach in class, I talk a lot about arms: how to engage the back muscles, avoid lifting with the shoulders, keep a shape to them, etc.
But placement is critical. If they are too low (i.e. at your waist), they drag you down and make you collapse in on yourself. If they move and if you can't maintain their placement as you turn, you lose the aerodynamics that comes from the centrifugal force of your body moving around a pivot point.
Typically, an unsuccessful turn looks like this: a dancer does a lovely demi-plie in 4th position, arms are stretched out and to the sides, and then when she goes to releve, they drop down. In other words, as she rises, the arms lower.
No. Don't do that.
Instead, bring your body up to meet your arms.
1. Find the placement of the arms opposite the center of your breastbone (usually this is the ideal position but yours may vary slightly higher or lower) and hold them in a round 1st position or slightly crossed if you are doing Balanchine style turns.
2. Take your 4th position demi-plie or long lunge (again, if you are studying Balanchine or are doing an en dedans pirouette) and stretch your arms as usual.
3. When you go en releve, you bring your leg to its position (retire, attitude, etc.) and rise up to meet your arms. Do not drop the arms as you go up.
Easy-peasy, yes? Ha! Never. Turns are never easy but this small change in your arms will make a BIG difference in your turns. You will not collapse the abdomen or hips; you will not drag down your body, weighing it down so the turn won't be completed; and you will keep a long straight spine which aids tremendously in multiple turns.
Hope this helps! Happy dancing~