Crossbleed Valve  -- Function

Bleed air is hot, compressed air taken from the engine compressor, which is used as an important source of energy for the aircraft. 

The bleed air contains usable energy in the form of its high pressure and high temperature.   Bleed air can be used within an aircraft in many different ways - 

• starting a second engine and/or an auxiliary power unit/electrical generator.

• powering pneumatic actuators and servosystems. 

• deicing the engine and wings.

• maintaining cabin pressurization and environmental conditions.