What Causes the Spinning Pipe Effect?

I won't give you the answer directly, but the setup described below will help you figure out why only one letter is visible when the pipe is spinning.

With the following setup you can cause the pipe to move slowly in the same way that it moves while spinning freely.

Drive a thin nail partially into a 4"x4" block of wood leaving about 3/4 inch sticking up. Cut two short pieces of 1" OD PVC pipe. Plug a cork into one end of each pipe. Connect the pipes with an on-center needle, leaving an 1/8" gap between the pipes. Press down on the "O" end of the cylinder and roll it forward against the nail, keeping the end nearest the O from slipping on the wood. Notice the velocity of the X and the O with respect to the block of wood as they rotate to the top.

Additional food for thought:

Why does the pipe, after bouncing around a bit, end up spinning in one place, with one end rolling on the table without slipping?