BLEED AIR DUCTING --
As a licensed engineer on the b737 ----------- ,it is to do with the electric distrubition of the electrical system of the aircraft when you start the engines -------with no apu available ----------------------------- if you start the right first , if i remember correct--- some instrument transfers units to the capt side dont kick in automatically ----------------------------- on some aircraft starting procedure can depend on which engines are fitted with hydraulic pumps say the bae 146 for example -------------------- on the b737 , the external air cart is connected to the right side , boeing designed that the left engine is always --- normally---- started first-----------if you have apu air ,you wont it to go to the left engine and not to the right engine by having the bleed air crossfeed closed ----------------------------------- i have had one time the external starting air connection flapper failed and to get that engine started , external air cart was connected for start , and after starting the no 2 engine bleed and the crossfeed valve closed and both on mel [ not sure if there were pax ]