where did the shut off dot go?

[radio nut]

I did have a CBS-Columbia set with a 21yp4 crt that when shut off would go dark instantly......but if you waited for about 10-15 seconds would have what seemed to be lightning on various spots of the crt on and off for another 20 seconds.
It was kinda like watching a thunderstorm!
I sold the set to a CBS-Columbia collector who had all their sets but the one I owned.
I still kick myself for not getting video of it!