Have a look at FCOM 3.02.29
HYD Y ELC PUMP LO
PR or OVHT
It is suggesting that it can be used in the event of a PTU and Y ENG PUMP failure. I haven't seen though where the procedure is written to turn it on in the first place. Vol 1 implies that it can only be used on the ground but that is a bit of red herring. I look forward to the definitive answer.