This is a really simple little trick. Its more common sense than anything but it might be useful for someone :)
It's basically for when you need to run a device but don't have enough batteries and can't be arsed to goto the shop to buy some.
So for this example I'm using my electric shaver but a more practical or likely example would be a battery radio.
Step 1: Get a Metal Object
Get a metal object, ideally something like a large pair of nail clippers is best since you simply open them up and squeeze them between the battery points where you have a missing battery. It's that simple, the device will now run off just the one battery and you can sit back drinking beer rather than going to the shops.