[Gurkha]

Most of today's cars like Honda City, Accord, Corolla, Palio, Optra have knock sensors, which is nothing but a sophisticated and insulated high quality microphone attached to the engine block. What it does is monitor for engine knock and then tells the ECU to retard timing, leading to loss of power, using high octane fuel is one good way of assurance that knock won't occur, specially on hot day with a/c running and vehicle heavily loaded going uphill. One of the reasons for better mileage with high octane fuel is due to the engine running at its full efficiency rating and thereby developing its rated power. This means that the driver doesn't have to hold the throttle for longer duration to get to his/her desired power.

Same thing with additive added higher cetane diesels like Turbo Jet etc. Not only do they keep injectors and combustion chamber clean, the high cetane ensures better burn preventing power robbing knock, specially in turbo diesels.

One thing I would like to point out, if your vehicle suddenly develops knock using your regular brand of fuel, then using higher octane/cetane is stop gap measure, you have other problems looming like incorrect timing, bad plugs, clogged inkectors and most likely culprit, carbon build up inside the combustion chamber.