Although it makes some sense to rev the engine of the donor car (more revs = more power = more electricity, right?), in actuality it does very little to help starting the recipient car.

Basically the alternator of the donor car cannot provide near enough current (about 65 amps on average, over 3000 RPM) for the short period of time required to start the other car. The donor car's battery therefore is needed to do the majority of the work (hundreds of amps of current are needed over a second or two). The donor car should running to allow the recipient car's battery to charge up a little before trying to start.

If the first try is not successful, simply wait a minute or two for the donor car's alternator to charge the recipient battery a little, then try again. Repeat until started, or bored.

I would recommend detaching the jumper cables as soon as possible after starting the recipient car to avoid a fire hazard. Once the car is started, one should drive it for a while (ideally 15 minutes or more after warming up) to allow the charging system to bring the battery up to a reasonable level so it will start again the next time.

Lastly, do not rev the engine of the recipient car until it has had a chance to warm up. The majority of the wear on a car's engine is in the first few moments of start-up when there is little or no oil on most of the moving parts. So revving the formerly dead car immediately after starting may be satisfying, but will do much more damage than any other start-up and could cause more problems later on.

When you are jumping another car, your battery will start to provide current to charge the other car's battery as soon as the cables are connected. If you try to start the other car while its battery is still not very charged, you will have to provide not only the starting current for the other car, but the additional current due to the drained battery still charging. This is why you sometimes have to let it sit charging for a while before it will start.

Under this circumstance, if you rev the car providing the current, you will produce more voltage and current, and it will charge up the other car's battery a little faster. Obviously you wouldn't want to rev it higher than it would normally operate, and it will only charge the drained battery a little faster than it would otherwise, just by idling.

You do not need to have the car doing the boosting running at all. Some recommend turning it off to avoid a surge damaging the ECM.Therefore, revving the engine up is not needed. I have done both running and not, and have had no problems. I would caution against boosting multi battery vehicles from a single battery vehicle. The draw produced by a dead Kenworth with 6 batteries can overwhelm any passenger car (personal experience). My advice is get a portable booster box so there is no risk to your ride. They are worth it, I have used mine to boost boats, lawn tractors, cars, trucks, motorcycles, and RVs (!).

I've seen people rev the car because they think it does something more. But it really doesn't do anything. The power in your battery is more than powerful enough to start the stalled car without the other driver revving the engine. Once you do this, you should leave the two cars connected and turned on for a few minutes so that the stalled car can get some extra charge, but revving doesn't do anything. It doesn't really help or hurt. All it does is charge faster, but not enough to matter.

Revving up the engine during a jump-start is not a bad idea, within reason. You never want to "floor" the gas pedal, under any circumstances. But when your engine turns faster, the engine's alternator also turns faster. And when an alternator turns faster, it produces more electricity.