I liked this explanation of insulin resistance by Dr. Greger:
What if there’s enough insulin, but the insulin doesn’t work? The key is there, but something’s gummed up the lock. This is called insulin resistance. Our muscle cells become resistant to the effect of insulin. What’s gumming up the door locks on our muscle cells, preventing insulin from letting sugar in? Fat. What’s called intramyocellular lipid, or fat inside our muscle cells.
Fat in the bloodstream can build up inside the muscle cells, create toxic fatty breakdown products and free radicals that can block the signaling pathway process. So, no matter how much insulin we have out in our blood, it’s not able to open the glucose gates, and blood sugar levels build up in the blood.
Here’s a graphical representation of that, about mid-way:
He goes on to explain:
This mechanism, by which fat (specifically saturated fat) induces insulin resistance, wasn’t known until fancy MRI techniques were developed to see what was happening inside people’s muscles as fat was infused into their bloodstream. And, that’s how scientists found that elevation of fat levels in the blood “causes insulin resistance by inhibition of glucose transport” into the muscles.
And, this can happen within just three hours. One hit of fat can start causing insulin resistance, inhibiting glucose uptake after just 160 minutes.
Same thing happens to adolescents. You infuse fat into their bloodstream. It builds up in their muscles, and decreases their insulin sensitivity—showing that increased fat in the blood can be an important contributor to insulin resistance.
Then, you can do the opposite experiment. Lower the level of fat in people’s blood, and the insulin resistance comes right down. Clear the fat out of the blood, and you can clear the sugar out of the blood. So, that explains this finding. On the high-fat diet, the ketogenic diet, insulin doesn’t work as well. Our bodies are insulin-resistant.
But, as the amount of fat in our diet gets lower and lower, insulin works better and better. This is a clear demonstration that the sugar tolerance of even healthy individuals can be “impaired by administering a low-carb, high-fat diet.” But, we can decrease insulin resistance—the cause of prediabetes, the cause of type 2 diabetes—by decreasing saturated fat intake.
Fanatic Cook 
Short-term insulin resistance can be a beneficial, healthy response. When the body is in fat metabolism–as when fasting or when carbohydrates are not available–saturated fats signal the body to preserve the small amounts of glucose available for the organs that really need them, like the brain. This is especially important when glucose is so scarce that the body needs to make its own glucose from protein through gluconeogenesis. This makes it possible to survive weeks without food, and humanity might not exist without it.
This healthy short-term insulin resistance should not be mistaken for chronic insulin resistance, which arises in metabolic disease (and seems to be worsened by polyunsaturated fats). In healthy people, the insulin resistance vanishes quickly if you eat some carbs with little accompanying fat.
It might be good advice that one should not eat lots of fat and lots of carb in the same meal. But I don’t think it’s valid to conclude that saturated fat itself is inherently causing disease. Interestingly, both very-low-fat and very-low-carb diets can be therapeutic (though some people benefit more from one than the other). Fat and carbs together seem to be more deleterious than either one alone.
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