Originally Posted by Kingtal0n

Forget about turbos for a second. We will come back to that.

Take a naturally aspirated engine of any make model. Now, decrease the size of the exhaust outlet. You will find that as you get smaller and smaller, the exhaust temperature starts to rise at the choke. Also, exhaust gas velocity increases as the port diameter decreases. That is one reason why manufacturers choose a happy medium between flow (large size) and exhaust gas velocity/temperature (small size). Smaller exhaust outlets tend to keep the exhaust gas hotter, which tends to increase the flow of the exhaust as well. Thats one reason putting a large catback on a small displacement naturally aspirated engine tends to decrease engine torque.

Back to turbochargers.
The turbine presents a restriction. as long as it is shut, the exhaust gas MUST flow through the turbine. That creates HEAT and BACKPRESSURE which is used to drive the compressor... which also creates its own heat as the air becomes compressed.

If you need more help with explanation I would be happy to but ask yours questions from there before I go further.