I've heard pro's do it. I wondered how it's done. I thought you would have to get into some sort of contorted position to be able to do it. Well, I had to do it during my 32 mile ride home from my brothers birthday party.
I was hoping to leave the party around 6:30 in the evening so that I would make it home before it got dark. It began to rain around 6:00 pm and I was hoping the rain would blow over before 6:30. It never did and it looked like it wasn't going to clear up soon either. While I was waiting for the rain to clear up I got involved playing table tennis. I was holding the table tennis championship but after 7:20 I had to throw in the towel and get on my bike.
The rain was coming down as a fine midst and the roads were wet but I was able to ride at a hard effort pace. After riding in the park around 8:30 in the evening I had the urge to pee. It was dark and with no street lamps to light my way I wanted to get the "H-E- double hockey sticks" out of the park and onto the lighted streets. I did not want to stop to relieve myself so I thought if people figured out a way to do it, so will I. It may not be the right way but it worked for me.
(note: sorry, women have a disadvantage to perform such a feat.)
While coasting at about 15 miles per hour
With my right foot on the pedal in the forward position
I bunched up my right biking short pant leg way up.
I pointed my aimer straight down and all was relieved.
Ah! With a smile!
I know I need more practice to perfect my new trick but that will come in time. If anyone has any pointers please feel free to share them.