3T - it's a plastic roller sprayer thing that goes on the business end of a pressure washer - made by Karcher retails for about £70 (or £67 in B&Q

)
When it all goes together you spray a snow foam under the car using the detergent head then attach the circular sprayer to clean off said foam then revert to the detergent sprayer to use the wash and protect to get to all the hard to reach places !
Did the underside of Walter this morning in around 25 minutes using all of the provided samples of liquid. Quite a lot of road crud came off requiring a hose down of the drive way but I actually bought it for Mrs Hoppers Mazda 3 as she's a stables girl and the sort of "roads" she drives down weekly would make you guys cry !
typically given how beautiful a day it is out there she was still out at her works Christmas party so the 3 is still filthy !
To see it in action just type Karcher chassis cleaner into you tube ! It's your friend