Even though my new bestie 'lachlan' tried his best to scare me off from ever using this forum again... I will share an answer I received by Subaru NZ to explain my question... Hopefully this will be of use to other forum users... Thanks again to Subaru NZ for answering.
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Thank you for your email. I will try to answer your questions in understandable terms (difficult as the questions you’ve asked involve some fairly technical components, but I’ll do my best).
1. The car will roll backwards on a hill as at idle in a stationary position the transmission is effectively ‘decoupled’ from the transmission via the torque converter. This is good for fuel economy as there is no ‘load’ on the engine. As you apply throttle and therefore engine speed/torque, the torque converter transfers this drive gently into the transmission itself. There needs to be a specific rotational speed difference between the engine and transmission for this to result in forward motion (different depending on engine/transmission combination and engineers design). When the car is stationary, there isn’t the rotational speed difference to provide drive – this is the same on a hill, hence the ‘hill holder button’ which uses the parking brake system to hold the car.
Will the car roll uncontrollably down the hill? – difficult question. If the directions and cautions in the owners manual are you should use either use the park brake, foot brake or hill holder on a steep hill as using the accelerator to hold the car would result in a possible transmission overheat situation. I’d say that at some stage the engine/transmission speed differential would slow the car but this is not how the system is designed to be used and the onus rests with the driver ultimately to operate the vehicle correctly when stopping on a hill.
2. Difficult to answer this question exactly, but I believe the ‘bite’ you are experiencing is the ‘lock-up clutch’ inside the torque converter engaging or disengaging. This is the component that changes the forward motion of the car to and from a fluid coupling type (as described in the previous question), to a 100% mechanical type. Hope this makes sense? Not sure of another way to describe it.
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