HSBC, Hang Seng, and other HK banks had a series of special savings account offers when I lived in HK a few years ago. Some could be linked to the performance of your favorite stock or country's stock index.
Interest rates were higher back then, around 6% one year. What they were effectively doing is taking the interest you would have earned and used it to place a bet on the stock or index in question. Technically, one way this can be done, for instance, is with call options and zero coupon bonds or notes. But there was nothing to strategize with once the account was set up, so the investor did not need to know how it worked behind the scenes...
Looking at the deposit plus offering in particular, this one looks a little more dangerous than what I describe. See, now we are in an economy of low almost zero interest rates. So to boost the offered rate the bank is offering you an account where you guarantee the AUD/HKD rate for the bank in exchange for some extra interest. Effectively they sell AUD options (or want to cover their own AUD exposures) and you get some of that as extra interest. Problem is, if the AUD declines, then you lose money because the savings and interest will be converted to AUD at a contractual rate that you are agreeing to now when you take the deposit plus account. This risk of loss is also mentioned in the fine print. I wouldn't recommend this especially if the risks are not clear.
If you read the fine print, you may determine you are better off with a multicurrency account, where you can change your HK$ into any currency you like and earn interest in that currency.
None of these were "leveraged" forex accounts where you can bet on tiny fluctuations in currencies. Tiny being like 1% or 2% moves. Generally you should beware anything offering 50:1 or more leverage as a way to possibly lose all of your money quickly.
Since you mentioned being a US citizen, you should learn about IRS form TD F 90-22.1 (which must be filed yearly if you have over $10,000 in foreign accounts) and google a little about the "foreign account tax compliance act", which shows a shift of the government towards more strict oversight of foreign accounts.
