You are right. Insulin sensitivity is measured at the site of action, i.e. fat and muscle. Glucose causes secretion of insulin into the blood stream and tissue responds to the insulin. How well it responds is considered insulin sensitivity. There are many physiological processes involved in insulin release, transport, and eventual use by a target tissue. If your tissues are insulin sensitive, then they will absorb more of the nutrient enriched insulin. That is why bodybuilders are more into timing insulin release when their muscles are the most sensitive to it so we can cram more of it in. As far as when you are lean, or fat for that matter, and sensitive to insulin, taking in too many carbs/protein at the wrong times can cause insulin spikes at less than favorable times.