PRO-NOR-SUP-ENG P8/10
This is only for the Taxi out procedure.
You are turning the Y Elec Pump off before ENG 2 start, so no pumps are pressurising the Y HYD SYS.
The PTU runs a self test for 5 secs as stated, therefore isn't providing Px to the Y HYD SYS during this time.
So keeping it straight for 5 secs will allow all this to happen then the PTU will kick in to pressurise the Y SYS.
If you try and steer the a/c during this time, that will use HYD Px from the Y system possibly resulting in Low PX.
That's how I understand it.