not a teacher
"He sweeps (or swept) his arm out at the far wall of the pit"
"sweep your arm out"
As a phrase it generally means to move your arm away from your body in a swinging, or sweeping motion.
In your example, without more context, it's difficult to know if he was trying to touch the wall or gesturing towards it. Or possibly merely standing near the far wall when he swung his arm out. Is this the original sentence in which you found the phrase? Can you give us more context?