[bobcowan]

You need that bypass hose. Water is pushed through the block by the water pump. Then up the back, through the heads, and into the front of the intake manifold. If the hose is blocked, and the thermostat is closed, where would the water go? You're exactly right - nowhere. It just sits there and pressurizes the block and heads.

As the water around the heads gets hot, where can it go? Nowhere. The water is getting hot, but the thermostat remains closed because it's a long way from the heads.

The purpose of the bypass is to move the water in a complete circle with a closed thermostat: Pump --> block --> heads --> manifold ---> pump. Creates a nice gentle warming cycle, and eliminates hot spots.

Once the water in the closed circle starts getting too hot, the thermostat opens.