One of the things you can do with two-phase electricity (at least for those of us in the US that have 120-volt power as standard) is get 240 volts.

If you ever hooked a computer fan up to the 5-volt and 12-volt lines in an attempt to get something more than 5 but less than 12, that's exactly the way it works. A 5-volt line is 5-volts potential in reference to neutral ("zero" volts). Same with the 12-volt line. But if you use the 5-volt and 12-volt lines, the potential between them is 7-volts, and that's what the fan sees.

Because of the timing offset between phases there are times when one phase is cresting while the other is dipping below zero into the negative. Each phase still only develops a 120-volt potential versus neutral, but there are times where the potential between two phases becomes greater than 120-volts.

Now just because your home doesn't have any 240-volt outlets, don't think there aren't two phases present. Each phase can be used against neutral to get 120-volts, but you have to be very careful how the house is wired if there's any possibility that equipment plugged into different outlets might be hooked together somehow (A/V equipment especially). I learned this lesson very painfully in a criminally mis-wired rental property and my laptop and surround receiver suffered greatly in the process.

But if you know what you're doing and have a sane grip on electrical theory, you can do some silly things with this knowledge. For example, back at KEZI engineering, we were testing a translator that needed two-phase power to develop 240-volts for the transmitter power supplies. Unfortunately, there were no 240-volt outlets available in the shop. So the engineers wired a custom cable with two standard plugs on the end and a four-conductor two-phase connector on the other. They plugged the 120-volt ends into two different power outlets that they knew were on different phases and that's how they got 240-volts for the transmitter.

When that very same transmitter was installed, they set it up next to the existing translator which had a hard-wired 240-volt power conduit that they planned to re-use for the new rig once the old one was taken out. But in the mean time, they were up the creek because nobody had a four-conductor cable... only three conductor. The two engineers there pondered on this problem and finally decided to use the three conductor cable to get two phases (plus ground) out to the translator so it had 240-volts.

The problem with that was there was no neutral line. The power section of this translator used the two hot phases to make 240-volts for the transmitter, but also took each phase and put it against neutral to power standard 120-volt plugs for auxiliary equipment (exciter, telemetry, control panel, etc). With no neutral line, there was no 120-volt output. What they ended up doing (strictly temporarily) was to run an extension cord from a 120-volt outlet to the rack, put a power strip on the end of it and plug in all the 120-volt equipment to the power strip. It was a wonder it all worked!

I'm happy to report that once the old translator was stripped out, the new one was made to work on the existing hard-wired two-phase conduit and all that Rube Goldberg wiring was thankfully retired.