Probably a 737, but may be other types too. When the APU is not able to suply air to start the engine, a ground-air unit is plugged in. On the 737 it plugs in on the right of the fuselage centreline. It is used to start No 1 (the left) and then disconnected (safely, away from the running engine - which is at idle only). Pushback takes place and number 1 is 'run up' to a medium power setting to provide enough air to start number 2 using 'crossbleed air'.
Other types may start number 2 on stand due to the location of the air connection.
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one.