The circuit used in this demo is a simple potential divider consisting of an LDR (Light dependent resistor) and a fixed resistor. The potential difference across the fixed resistor is monitored on the CRO.
By facing the LDR towards the CRO screen, whenever the CRO trace arrives near the LDR, it illuminates the LDR and reduces its resistance. The potential difference across the fixed resistor thus increases. (This can be deduced by either (1) noting that the current in the circuit has increased or (2)the potential divider principle.) The CRO trace thus “jumps” over the LDR.
In a way, the CRO has ended up measuring its own brightness.