I was talking to our engineer the other week about why the PT6 gives hotter ITT starts as the battery starts to deteriorate. If you don't allow Ng to spin up and stablise before you introduce fuel, or if the battery is low and Ng doesn't spin up as high as it should, both will give hotter starts.
The layman's explanation was the reduced mass airflow between turbines meant less cool air was being introduced, whilst heat had more time to build up between the turbines.
You'll note ITT also rises in relation to torque as you gain altitude or on less dense days. With the bypass engaged, the incoming air mass would also be less dense than with ram-air - which has the same effect.
The difference is, with an intake restriction (bypass engaged), the engine is less efficient and has to work harder - burn more fuel (and create more heat) - to make the same power under the same ambient conditions. That's why ITT and fuel consumption both go up.
At higher altitudes, the engine has less mass airflow (higher ITTs), but is more efficient, so actually burns less fuel.
That's my rather non-scientific understanding of it
