Ergative Stage of PIE

Ergativity makes my hair stand on end. It forces you to look for
something significant about transitivity, something beyond the mere
fact intransitive verbs cannot be put into the passive. Then there's
the horror you get when you realize the usual distinctions between
subject and object don't work.

How's it go?

Man-abs kill = the man committed murder.
Man-erg kill bear-abs = the man killed the bear.
Bear-erg kill man-abs = Smokey smoked the man.

'The man murders' is spooky. You don't know if 'murder' is active or
passive (vs. 'the man sleeps' which is unremarkable). Maybe there
really is something deeply significant about transitivity.

It is often said that at an early stage, PIE, or better, pre-PIE was
an ergative language, a la Basque or Georgian. On the other group,
they have interminable boring discussions of Basque, but discuss
ergativity very little. Can ergativity be explained in 250 words or
less? 2500 or less?