Understanding overunder intervals I'm reading The Time Crunched Cyclist and trying to work out a training plan for a century later on this year. I am having a really simple problem understanding the overunder (OU) interval shorthand. For a ride listed as:

90 min. EM with 3 x 12 min. OU (2U, 1O) (8 min. RBI)


What does the (2U, 1O) designation for the OU interval refer to? 

I know I'm supposed to do 3 repetitions of 12 min. at some pace with 8 minutes in between. But what is that timing/pace -- how long under and how long over? I can't seem to find it explained anywhere in the text.
I had a difficult time understanding this too.  the 2U, 1O stands for 2 minutes under your threshold, and 1 minute OVER your threshold.  (that's a capital letter O, not a zero, which threw me for a long time).