RAY: A few months or weeks ago, one of my guys was under the hood of an old Chevy pickup truck, trying to find a vacuum leak. The way you find vacuum leaks is, we use a little wand that shoots propane. When the vacuum leak and the propane meet, you'll suck the propane in and the engine will begin to run smoothly, because now you're making the fuel-air mixture correct. And as soon as you take the propane wand away, all of a sudden, you've got far more air compared to the fuel than you're supposed to have, and it runs lousy again.

So there he is under the hood with the wand, and he's having lousy luck. A few minutes later, I see him doing something very interesting. He's pulling off the spark-plug wires and he's putting them back on, but on the wrong plugs. Two minutes later, I hear him on the phone ordering the part that he needs to correct the vacuum leak. 

How did he do it?