A pictorial and a video demonstrates how to build an invisible switch into a PC. A schematic is shown how to apply this technique for cars.
Step 1: How to Make an Invisible Switch in a PC Step 1
The first picture shows the reed switch which we will use. It's a glass tube with two contacts which are pulled towards each other making contact when a magnetic field is present. You can reed(read ;-) ) more details on wikipedia. http://en.wikipedia.org/wiki/Reed_switch
For the PC we'll use a read switch with a make contact.
The first step is to remove both side pannels from the pc housing by unscrewing the bolts shown in the picture. After that the front panel can be removed by pushing the front panel retainers together with your fingers to unlock them.
Step 2: How to Make an Invisible Switch in a PC Step 2
Now we can twist our read switch to the wires. Once the switch is twisted on, we can tape it to the front panel using some quality tape, so it won't let go. Mount the reed switch on a place where the front panel is not very thick, otherwise we may not be able to operate the switch.
Step 3: How to Make an Invisible Switch in a PC Demonstration
The reed switch is in series with the on/off button. This means the on/off button cannot be operated unless a magnetic field is present near the switch. You have to hold a small magnet close to the reed switch and push the on/off button. The pc will turn on and you can take the magnet away.
Some people are worried that the magnet causes problems with the hard drive. The field of a magnet at this distance from the drive is way too low to cause any problems. If you're still worried, you can place the switch as far away from the hard drive as possible.
Step 4: Protecting a Car
You can do this by glueing a reed switch on the inside of your dashboard and having some velcrow on the other side of the dashboard. Glue a magnet to the velcrow and you can stick your magnet to your dashboard activiting the switch.
Reed switches cannot conduct very high currents, so you'll have to connect a 12V car relay to it so you can switch larger currents. You can use this relay to break a vital wire in your car, like the contact wire behind your ignition key switch. You could probably also put it in your starter motor control wire or maybe your engine management, but you should check this with your garage for your specific car model because engine management systems can be sensitive.
The schematic shows how to connect it. I'm showing a reed switch with an alternating contact here, this can be usefull, although the relay usually also has an alternating contact. You can choose if you want the car to run with or without the magnet by choosing another contact on the reed switch or the relay.
Thieves usually do know all the standard alarm systems and protection systems but will have a hard time locating this protection since they don't know what to look for.