No learner, I used to wonder about that (I have traded HSI for the last 3 years) but its not that. SPI for example is a 1 point index (and STW is 0.1 points) and both trade in an orderly manner.
Also, HSI doesn't have substantially less daily volume than SPI or STW (obviously much less than the 3 nikkeis).
I am told by someone who has spent more time in Asia than I that its also not "chinese gambler's" as has, somewhat racistly, been suggested here before. The behaviour of a contract is substantially affected by the order matching system that is used and apparently HSI's one is a little different. That may be why MHI is still not on the permitted list for US citizens.
Until the changes in August normal gaps were modest and a 20 pt gap was the exception and, if experienced, likely to be associated with being wrong at yesterdays high. Normally you could fill a few contracts within a few points.
The changes corresponded with the opening up of HK to some organisations within Mainland China. I am told that a bunch of well financed folk out of Shanghai took up trading the HSI at that point in practice for a new Chinese Futures contract. That group were big enough to make the existing locals lives difficult and the intraday volatility changed as a result.
I don't know if that's 100% true but someone else who runs a trading organisation that participates in HSI in a big way told me initially that he'd seen it before and it would be back to normal in a couple of weeks ... and then a month and a half later "this is insanity." At that point I decided it would be more useful to try to adapt my strategy to the other main Asian indexes than to deal with prices that jumped right off my dom
I must say that going from an environment where trades lasted a few minutes to one where they go for half an hour or more has been interesting after adapting to the old HSI. I keep an eye on it to see if it drops back into "dom wide" ranges.