excuse my ignorance but whats this disconnecting the hose from pressure washer all about?
Two main reasons. After washing the car, the final rinse should be done with an open ended hose rather than a spray (jet washer or hose spray head). So you need to disconnect the hose from the pressure washer, and remove the stop connecter (if using one) to get a non spray flow of water from the hose.
The reasons the final rinse should be with an open ended hose is, working from the top of the car down, open ended rinsing allows the water to 'sheet' off the car, thus reducing the amount of standing water left on the car so that
1) less water to dry of with a MF towel - which reduces the need for towel contact and,
2) less standing water reduces the risk of water spots forming.
Water spots not an issue if you have very soft water or use a water filter attached to your hose (as I do) but for those in hard water areas without a filter, if there is a lot of water left on the car after the rinse, water spots can form very quickly, (quicker than you can get round with a drying towel) especially if the car is in the sun, and they can be very, very difficult to remove, sometimes having to be polished out with a DA. Using a spray-on rinse aid at the final rinse can help, but the best way to leave the minimum water on the car is an open ended hose rinse. A final rinse with a spray or pressure washer lance leaves lots of water droplets on the car, each one a potential future water spot! There are products to remove water spots such as CarPro Eraser but their effectiveness is questionable so best to avoid getting water spots in the first place.
Equally important - less standing water on the car after rinsing equals less towel drying which means less 'contact' with the paintwork, so less risk of swirls and marring.
That's the full on OCD reason that disconnecting the hose from the PW is important. Bet you wish you'd never asked.