The E34 is the old one? You're probably talking about a reverse flush in that case. You do it if you're worried about bits of crud in there, but I'm not sure if there's much point. The way I remember it, you disconnect the bottom hose, open the heater water valve, make sure the themostat isn't stopping the flow, then connect a garden hose securely so that water can be forced up from the bottom of the radiator and emerge from the bottom of the engine, flowing the opposite way to normal.

I'd disconnect the top hose and reverse flush the radiator first as this is most likely to harbour flakes of rust and stuff and you don't want to force it into your engine.

I don't really see the point though. If you're engine's overheating you probably have other issues.