Sorry for the delay bro.. I didn't know u were so desperate to know . It was indeed a good question for which I also had to consult few guys who have better knowledge.
Well, actually that day I was in a hurry n soon after the catalytic was fixed, I left the workshop, otherwise it could have been resolved the same day.
The reason for engine check light was the O2 sensor which is responsible for sensing the oxygen saturation. There r 2 sensors (one before n one after the catalytic housing) which feed the ECU and they control how much extra oxygen gets pumped into the exhaust stream by sensing the difference between readings of both sensors. Maintaining the correct amount of oxygen makes the reduction and oxidation reactions more efficient. When the catalytic converter is cleaned using chemicals and water etc and fixed back and then we run the engine, it heats up and reacts differently (compounds mixed with steam etc). Resultantly, the O2 sensor goes haywire and gives a signal to the ECU that something is wrong because the sensor is confronted with an altogether different composition of gases. So, with the help of OBD computer, the ECU is told ALL IS WELL and then the engine check light vanishes. Even with the engine check light ON, there was nothing wrong with the driving of car but of course we don't want it to remain ON.
I hope this makes some sense. For a more technical description of this phenomenon, other members can shed some light.