LEM ,as AT said in case of bleed trip off light you'll have to select also affected side pack off- so no 2 packs on only one engine bleed. That's qrh.
The valve will open automatically when you select bleed off-when you select manually the switch to off-it won't trip by itself- so you'll have ram air for systems that require it i.e. hydraulics pressurization,water,etc.
Due to high bleed load it is not recommended to use wing antiice,it can result in the second bleed to trip off --loss of presurization.
It is stated in the qrh that you should avoid iceing conditions for this reason.
Brgds Alex