School me on transmissions, overdrive, and lockup converters

[wilkin250r]

I've done my research, I know the basics. I know the planetary gears in an automatic tranny, I know how torque converters work, lockups, ect ect.

What I DON'T know are the specific details of how things go "bad".

Don't tow in overdrive. Everybody says it, but what is the reason? Does it cause the transmission to hunt for the right gear? Too much shifting can be a problem. Does it cause excessive heat? Can I tow in overdrive if I have an external cooler (and temp gauge to monitor)?

How about a lockup converter. Why does it only lock up on OD? What happens if I have a manual switch to lock it, but I never switch it on? Is it the same problem, heat? Why can't I tow with my converter locked?