Yeah so spoke with my mate who's always in the show and shine, basically said usually a source of water that can be used at the shows to fill a bucket up and wash the bottom half of the car to remove any road grime picked up on the drive down to the show from home. Top half gets The quick detail spray treatment, then some will polish again, though not often.
Day before travelling to the show they'll give the car a full detail, hence why they only wash bottom half of the car and quick detail the top half, as top half won't need washing again.