London Blumpkin

In this modern iteration of a traditional Blumpkin, party1 sits reverse cowgirl on the toilet bowl, fellating party2 while they deposit an upper-decker into the tank. This gives the appearance of riding a double-decker bus. If both parties are simultaneously pooping, the terminology can be upgraded to "Royal Flush".

I heard the Queen used to give out London Blumpkins at Buckingham Palace. One lucky guy even got a Royal Flush!