BSI Un-coupling 

In this video you can see two units equipped with BSI couplers, dethatch - un-couple.
When the driver presses the un-couple button with the train in the correct configuration, an air pressurised piston pushes the uncoupling bar out. This then latches under the latch pin. In this position, the coupling pins that were forming the mechanical connection have retracted into the housing - allowing them to slide past each other as the unit sets back.
There is a "pop" as the pressurised air connection of the two air seals disengages.
The electric boxes retract upwards again to protect the electric contact pins underneath.
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