Not only do they check the color of you fingers for refil time (the time it takes your blood to return should be less than 2 seconds) but they check your oxygen saturations. That is the amount of oxygen that is in your blood. It should be 100% or just under (especially if you are a smoker). By doing that they can check to see if you need extra oxygen. The machine doesnt work properly if you have polish on, or falsies.
Hope that makes sense,
Serena X