My understanding of ISF theorem:

All hands that are better than 50% of your opponent's range can add some value to your betting range. The more of your opponent's range the hand beats, the more value it adds. For every set amount of "value" in your value betting range, you can add a certain number of bluffs. If your value range is strong enough, it can allow you to bet your whole range. Hands that beat a medium % of your opponent's range add little or no value to your value-betting range but they do add value to your checking range. If your range contains a lot of these kinds of hands then it can be better to use the stronger portion of your value range to bolster this checking range.

Interesting to see that no one seems to be submitting examples of when they checked good hands in a weak range because of ISF theorem. They're all about betting bad or medium hands in a strong range.