Pete.
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- Kent, UK
Thanks, still don't understand, but don't worry about it.
When the motor is first started K1 and K2 are closed. This joins the 3 green ends together to form a star connection and vastly reduces the motor starting current (because each phase to phase has a pair of windings in series doubling the resistance).
Once the motor is up to speed K2 opens and K3 closes disconnecting the star point and re-connecting the 3 windings in delta (effectively joining the 3 pairs of green.orange circles together in the diagram). This reduces the total winding resistance (because they are now connected as a parallel pair) which allows more current draw and more power from the motor.
The most likely cause of the bang is that K2 didn't open when it was supposed to which meant that closing K3 put a dead short across the supply.