bypass hose

Bypass hose is just what it says - bypasses the radiator when the thermostat is closed. Gets the engine to come up to operating temperature quickly while allowing the coolant to circulate. No circulation means no warm coolant flowing at the stat and can cause hot spots and boiling coolant before the stat opens.

No stat or an open restricter - remove the bypass so all the coolant flows through the radiator. Stat - you need the bypass.