It's not this:
|Georges Seurat 1888|
Many students think of rond de jambe en l'air as a leg circle in the air (again, NOT the can-can) since "rond" translates as "circle" - for dancers, at least. But the shape you are drawing in the air is not exactly a circle. It's more like a flattened oval.
|The path of the rond de jambe en'lair en dehors|
Conversely, the proper way to perform a rond de jambe en l'air en dedans begins with a battement a la seconde, then the toes draw a half-circle or oval to the front of the knee of the standing leg and then go straight back out to a la seconde.
2 very important things to remember for this step:
1. ALWAYS fully straighten the knee on the recovery to a la seconde, even if you are going to do more than one rond.
2. Maintain a strong straight support leg and avoid sinking back into the hip of the support leg.
One more tip!
NEVER look at your leg while you do this step. The mere act of turning your upper body to let yourself watch the leg twists the hips. If you can't see yourself in a mirror, then use your peripheral vision. Better yet, just feel it working properly.
Hope this tip helps you! Happy dancing~