IB will convert base currency to HKD as required in the trade and your profits or losses will remain in HKD. I don't think they convert for margin as thats just a "safety measure"; more a loan than a reality. You only need to make conversions when getting a check for your profits later.
by the way... anyone's thoughts here on 19300-19400 being a support base before another leg up? or are we back down to 18750 test first? i say 19300 is the near term bottom, since that corporate tax proposal reducing tax burden for chinese shares 25% caused the spike up from current levels to 1500 pts above.
HSI /HHI is certainly influenced by mainland china stock index which has tribled in less than one year from 950 to 2850 points. mainland china will launch its first stock index futures contract in a couple of months.
been tracking Shanghi SSEC, it can move 2% in 5 ot 6 min, this is going to be fun or hell for traders...