When running a train, it is common courtesy to paddle the ass after pumping it full of jizz. Just a couple of hard smacks create a nice rosy glow and tightens the hole back up for the next guy in line.
There is nothing better than sliding into a really tight hole that is pre-lubed with several loads of slick, creamy sperm. Churning the previously deposited cum loads into a frothy foam, while loosening up the hole with your cock feels great.
As a bonus the tightening of the hole due to the paddling makes sure the cum stays inside.