What's behind "Negative Balance Protection"?

 

How is it possible that a broker (regulated in Europe) can protect a retail client from negative balance unconditionally, whatever is his loss (below zero) when using leverage, because this is what many brokers state: unconditionally. I mean what's behind? An assurance that guarantees a refund? Or it is technically impossible to lose more than you have on account because there are some processes behind that guarantees, like if the trader in reality is virtually trading? I'm not talking of stop out, that is a measure to avoid negative balance, but I'm talking about when the negative balance happens...it would be interesting to know how it works technically behind...someone knows? I will ask to my broker too, but I'm also curious to know if all brokers work the same...

Reason: