In order to trick Darlene into believing she was the one distorting space to curve the rocks, Leon had purposely hidden the fact that he could also manipulate the Spatial Laws.
He could have chosen to demonstrate it to give Darlene a feel of what manipulating space was all about.
But if he had done so at the start, it would only prove that he can do it. It did not necessarily mean that she can too.
If she had such thoughts and failed, it would no longer be possible for him to trick her then. She would believe that every space distortion was caused by him and not her own abilities.
"Eh? Already? Since I manage to manipulate space, how about you let me off and let me practice on my own?" Darlene requested.
She still had some fear of rocks being hurled at her.
Surprise 2nd chapter of the day!