Thanks @kpax. I found this in the fact sheet for BBUS:"BBUS uses...

Thanks @kpax. I found this in the fact sheet for BBUS:
"BBUS uses futures (i.e. S&P 500 futures) to obtain a magnified (or geared) short exposure, rather than the underlying shares. As the futures market closes at a later time to the share market, at times BBUS’ performance for a given day may differ from that indicated by the share mark t. As theU.S. futures market is open during ASX trading hours, BBUS’ performance during the day will reflect movements in the futures market."
On the other hand it also says this:
"A 1% fall in the U.S. sharemarket on a given day can generally be expectedto deliver a 2% to 2.75% increase in the value of BBUS (and vice versa)."
Oh well...