Read here to learn about riddling of Champagne
Riddling is the English word for "remuage" which is the word for turning or shaking. In this context it means turned which is what you do which the Champagne bottles. Here you can see the process of riddling:
At first the bottle is flat, that is to have the most contact possible with the yeast, which is the sediment that you see on the bottom of the bottle. As time go they will slowly be turned so they at last face the floor. Which means all of the yeast is in the top of the bottle. There is 2 ways to do riddling.
The first way is the traditional way, which is done by hand. Back in the day there was people who did this all day, just going around in the Champagne cellars and turning all the bottles by hand. The traditional method is seen on the photo below, where you can see the riddling racks filled with Champagne bottles. And if you look on the left from the riddling racks you can see Champagne bottles aging, that is the process just before they will be sold.
The second method is riddling by machine, which is a more modern and untraditional method of riddling Champagne. Using machines for riddling will get you a more precise riddling, but it is not very different from riddling by hands. If you look on the photo below, you can see the riddling machine on the right.