Agree with xkoote,
in the NORM position you provide maximum bleed air to the pneumatic system, therefore it's the required position for engine start (anyway a modification was planned to automatically shut-off the economy mode valve regardless of the position of the switch, in order to prevent an hot start).
In the ECON mode the bleed air available for the packs it's reduced, therefore you have lower EGT and lower fuel consumption, and it's supposed to be used when the reduced flow it's sufficient to maintain the cabin temperature requirements.