A heater bypass is an absolutely critical component in a Subaru conversion if you intend to use a heater which throttles the coolant flow through the matrix to control the temperature. Note this includes the standard heater system in the T25 / T3 / Vanagon. Without such an adaptor, your engine WILL OVERHEAT.
  The heater bypass works by allowing an amount of coolant to bypass the heater when the heater valve is closed.