I agree this is something that can ruin an engine if done too aggressively. But with a disciplined hand it can clean out an old engine.
The premise is the same as what you find with a leaky head gasket....when coolant gets into the cylinders, one symptom is a clean combustion chamber (or clean spark plug). The water "steam cleans" the chamber, resulting in a cleaner engine.
95 E320 Cabriolet, 131K