first, a little background on insulin. insulin is triggered in response to various things that enter the blood that cause an osmotic imbalance between the blood and the cells around it. for example if you've taken high school biology you'll remember that when a semi-permeable membrane separates 2 identical solutions these solutions are said to be isotonic to each other. when you put something like glucose in one side then this side is hypertonic to the other side. as a result water will diffuse into the compartment of higher solute concentration. glucose is too large to move across the membrane. one side reduces in volume while the other side increases in volume.
this is the same thing that happens in the body. when a lot of little molecules like glucose enters the blood it creates an osmotic imbalance, but if the build up of glucose in the blood is too great then too much water will be lost from the cells and end up in the blood. this causes massive problems with pressure because a certain volume of arteries and veins can only hold so much blood. the increased pressure will go into the brain, eyes, other extremeties, causing comas, cataracts, possibly death. insulin transports these molecules, glucose, aminos, fats, creatine, etc. from the blood into the cells, thus eliminating this osmotic imbalance. electrolytes play a much bigger role in fluid tonicity, but they have their own channels in the membranes and can move freely whereas larger molecules (glucose, aminos, etc) must have a transporter (insulin) to shuttle their movement.
now that you're familiar with the function of insulin i'll explain as uncomplicated as i can about sensitivity/efficiency and how it relates to type II diabetes.
when your body releases insulin it needs to bind to insulin receptors to be activated and do its job properly. if these receptors don't turn on then insulin doesn't work. if these receptors are more active then it allows insulin to do its job better, thus requiring less free flowing insulin to be in the blood. this is insulin being more efficient, or sensitive. increased insulin efficiency means that not only glucose is transported into the blood easier, but aminos as well. to a degree fat will be transported better too, but fat molecules are huge compared to glucose and aminos and don't play as big a role in contributing to blood osmolarity, so increased insulin efficiency has less effect on storing fat as it does with storing glucose and aminos. it's actually the opposite because having a lot of insulin present in the blood inhibits fat oxidation. when the receptors are more sensitive to insulin then there is less free flowing insulin in the blood to regulate blood glucose/osmolarity, so this is advantageous for fat burning.
supplements that improve insulin efficiency will thus have an muscle anabolic and fat catabolic effect. chromium picolinate has a very mild effect on increasing insulin efficiency. IMO this is not worth the money because the effects are so mild that it's just not noticeable. vanadyl sulfate is another one that increases insulin sensitivity. this is more effective than chromium, but is slightly more toxic at doses high enough to have a significant effect. vanadyl used to be one of my favorite supplements, but toxicity concerns have led me to not use it anymore. glucophage, brand name metformin, is a more effective drug that has less sides than vanadyl, but is only available by prescription for people who have type II diabetes. these people have poor sensitivity of their insulin receptors and are sadi to be insulin resistant. metformin is used to boost it without causing the patient to go hypoglycemic. metformin has a very serious side effect in about .1% of the population that suffers from lactic acidosis.
the opposite of insulin efficiency is insulin resistance. this is when the receptors refuse to turn on such as the case with type II diabetics. obesity is a heavy contributor to type II diabetes. there are some anabolic steroids that cause insulin resistance too.