juggernaut

In getting ready for my next show, I did something completely different this year under the recommendation of my trainer. I carb cycled and took two days out (yesterday and today) to empty my glycogen stores to a bare minimum. I think I am flat because usually, i am able to get a ton of veins pumping throughout the bodypart being practiced during posing. Today, very small amounts of veins popped. No matter what i did. Is this a result of the glycogen being depleted? No matter what I did, I couldn't get anything going.
What exactly is the term defined as being flat? Why should one do it? I kind of know the answer, but want to hear from another source.