Your not understanding doesn't means it is not true.

3rd Debate: Obama Outpoints Romney - Eugene Robinson, Washington Post

“But going into the last two weeks of the campaign, we still have no idea how Romney would manage to cut income tax rates by 20 percent without increasing the deficit. We don’t know which tax deductions he would target for possible elimination, although he did say at the town-hall debate that his plan might be to establish an overall deductions cap and let taxpayers decide which ones to take. He still has not made the slightest attempt to demonstrate that the arithmetic adds up.”

If Eugene only used tools he understood, he would never ride in a car or boat (with a motor). I believe the fact that Progressives don’t understand how Mitt’s plan would work is a clue to why the last 4 years didn’t work. They don’t have a clue how the economy works.