built-in gains tax

I've been reading a few net articles on the built-in gains tax issue that arises when a C-corp converts to an S-corp.
I just don't get it. Most of the writings are written as if the author
is talking to another accountant instead of newbies. I can just see an accountant or estate lawyer reading the article and nodding their heads in approval, while others like me read it and are thinking "huh?!". The funny thing is, the only people that have a need to even read the article are those same people that have trouble understanding it becuase the author learns or knows something about it, and starts talking at a level above the newbie.
A partner and I started a C-corp many years ago before LLCs became popular. We have a small retail location that rents to 3 small retail tenants. When the depreciation ran out, we began to incur hefty double taxation, so we converted to S-corp a year ago. Eventually the partner and I will split up in the near future. We have it on informal advice that if one of us sells to each other, there is no built in gains tax (the buyer just picks up the liability for it). If we both, in concert, sell to an outsider, then we face a hefty tax... same for if one of us sells to a separate organization of ours (If I buy his half, I may start a new LLC and get rid of the S-corp).
So can anyone give a simple, yet effective and informative tutorial on the built-in gains tax? Talk as if you were talking to a college freshman taking his first Business Basics class (the earliest in a string of business class where they learn what a sole prop vs partnership is, etc)
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