I have a late model Husqvarna with the wash nozzle factory installed.
In theory, the water turned on and the blades engaged, it's supposed to wash out the deck bottom .... but, it doesn't work very well.

. I have done it a few times then, jacked up the mower and wash out all the remaining clumped grass. The system on mine misses a good 75% so, I quit using it as it's a waste of time for me...