Hi cheertiff710! Ketosis is a state in which your body is burning fat for energy rather than glucose. That is the purpose of limiting carbs to 20 when on induction. Once your body burns off the glucose stores (from high levels of carb intake), a proces which usually takes around 48 hours or so, it switches to burning fat for fuel. When the body is burning fat for energy it expels ketone bodies in the urine and breath. That is why some people use Ketostix to test their urine. If the stick changes color it is "proof positive" that one is in ketosis. You'll also hear people say their breath has a certain odor when in ketosis which is "proof positive" to me. I'm not able to instinctively tell if I am in ketosis or not ~ I need to use the strips.
Here's a
link (Ketosis & Ketoacidosis) to more info.
Hope that helps!