In order to describe conditions inside, different parts of the engine are referred to by numbered stations. Station 1 is the air inlet. Station 2 is the entry to the compressor. Station 3 is the exit from the compressor. Station 4 is near the exit from the combustion chamber. Station 5 is between the compressor turbine and the power turbine. I believe station 6 and 7 are respectively the exit from the power turbine and the exhaust port, but I'm not sure. An additional station 2.5 is defined between the axial and centrifugal stages of the compressor.
At both station 2.5 and station 3 compressed air is deliberately bled from the system. You'd think after going to all the work of compressing it, you wouldn't want to let any out, but you do, for two completely different reasons.
At lower power settings, the axial compressor is more efficient than the centrifugal one, so at low power the axial compressor delivers more compressed air than the centrifugal compressor can accept. That would lead to a buildup of pressure at station 2.5, creating a back pressure that would cause the axial compressor airfoil blades to stall, completely disrupting the calculated flow of air through the engine.
So the manufacturer has included at station 2.5 a compressor bleed valve which automatically opens at low power settings and closes at higher ones, becoming closed at about 80% NG. The pilot has no way of controlling the valve, and the only action she has to take is to allow the engine speed to stabilize at 85% NG for five seconds before applying takeoff power, in order to allow the bleed valve to close smoothly. When the compressor bleed valve is open, compressed air is just dumped sideways out of the engine into the atmosphere.
If the bleed valve were to become stuck open, the pilot wouldn't notice it during ground operations preparing for takeoff, because it's supposed to be open then. When takeoff power was set, the pilot should notice that engine torque is lower than it should be, but that NG and the temperature at station 5 (T5) is increasing, as the fuel control unit increases the fuel flow in a fruitless attempt to increase the torque. It's never going to get there because the air it needs to burn is being dumped overboard. If both pilots are too busy looking out the window and thinking "I love this job" (new first officer) or "I hate this job" (disgruntled captain) and miss the engine indications, the result could be a compressor overspeed, which would wreck the engine. The opposite problem, of the valve closing at high power but failing to re-open at low power is apparently very rare on this engine, but would result in a compressor stall.
Meanwhile, at station 3, compressed air is bled out of the engine to be put to work. Air is routed from the engine through DC electric shut-off valves into a duct in the roof. At cruise power, bleed air is at about 450 degrees C and 80 psi. It is used as-is if the heater is in use and/or routed through a heat exchanger, air filter, and pressure regulator to provide cooler 18 psi air for pneumatic systems.
The switches that control the bleed air supply actually only control bleed air available to the heating system, airframe deicing, and the pneumatic autopilot. Air for deflector operation and, in older models, the pneumatic flight instruments is obtained before the bleed air valves, so that the deflectors and flight instruments work even with the bleeds switched off.