How should the transmission be set to back flush the engine?

The correct setting for back flushing the engine is flush mode. This setting is specifically designed to allow the engine cooling system to be cleaned while water is circulated through it. When properly set to flush mode, the engine's water intake system can draw in water from the flushing device rather than the water from the lake or ocean, thereby allowing debris and contaminants to be expelled from the cooling system without the risk of harm to the engine.

In contrast, neutral does not facilitate the desired flow of water necessary for back flushing. Reverse could potentially disrupt the flow or cause damage to the system by creating back pressure, while forward typically operates the engine under normal running conditions, which does not suit the requirements of flushing the cooling system. Setting the transmission to flush mode optimizes the effectiveness of the back flush, ensuring that the engine cooling system is adequately cleaned and maintained.