There's a YouTube showing this dolly (or one very similar) here:
The pivot mechanism is shown at around 2.20 on the video. The pivot is just a bolt going through a length of 4x4 box section, which to me seems a bit minimal - but then I'm just a wanabee engineer as we all know. On my own tow dolly the vertical weight of the car was taken on a large greased plate underneath the pivot, a bit like a 5th-wheel coupling.
Thanks Cody for starting this interesting thread - sorry I ruined it by asking a question. I'll try to remember the 'just take it for what it is
' rule in future