One word "pseudoscience".
That may count as two words, but the meaning remains the same.
None of the guys who sell these gizmos can explain how you can get more energy out of them than you put in. If you read their websites, you'll see that they are all full of sound and fury, signifying nothing. When they try to explain how the device works, the description descends into anecdote and assumption.
The laws of thermodynamics state that the energy you put into a process (say splitting water into Hydrogen and Oxygen) limits the energy you can recover from the reverse of that process (burning the Hydrogen and Oxygen, to get back to water).
Until someone can tell me how it works, I won't waste my time building one.