I owned an HP9000 which works the same way.
The "Wash Printhead" makes ink flow from the ink lines to the Capping station.
The difference I noticed between "PH Clean" and "Wash Printheads" is mainly that when cleaning printheads the solenoids located in front of the capping unit make cappings to first taking ink and then taking air, leaving the capping unit and tubings "dry"
When you wash printheads the solenoids only make ink to flow for a small period, leaving it in the capping units, for "washing" the printhead's noozles.
You should leave ink all overnight soaking the printheads (recommended in the manual). You most have noticed that after doing this wash, the printer performs automatically a "Clean PH" procedure.
I used to run several "Wash PH" after changing dampers, for instance; forcing ink to get down to the Printheads.
I hope I made myself clear. My english isn't as good at it should.