These old-fashioned inverters are the cheapest to make, but the hardest to use. They just flip the voltage from plus to minus creating a square waveform. They are not very efficient because the square wave has a lot of power in higher harmonics that cannot be used by many appliances. Synchronous motors, for example, use the 60Hz component and turn the rest of the frequencies into heat. The modified sine wave is designed to minimize the power in the harmonics while still being cheap to make.