How does it typically happen – from a frontal impact ie extension at the top of the headtube/compression at bottom, or from, say, casing a jump causing extension at the bottom of the headtube/compression at the top?

I’d always assumed it was the latter, but on the angleset thread there seems an assumption that it’s the former.

Ive snapped a headtube off casing a 20 foot double. I came to still holding on the the bars, even though the rest of the bike was quite a way away.

To be fair, the previous day I had hit a gate that “jumped out” at me on a descent in the peak quite hard.