Hit the borrow button in top right under the on the exchange tab.
borrow the bitassets and buy BTS?
no .. the other way round .. you borrow bitUSD (or bitCNY ...) using BTS as collateral
Both are correct... or am I wrong?
1) Opening a short:
You borrow "the bitasset", "bitUSD" (using BTS as collateral) and then buy BTS (AKA sell borrowed "bitasset" for BTS)
2) Closing a short:
You buy back what you borrowed, hopefully for less BTS than you sold it for, and hit the "close" button on account view to give "the asset" back to the blockchain
3) Profit?
If you (1) sold higher than you (2) bought back, you have more BTS than before step n°1
If you (1) sold lower than you (2) bought back, you have less BTS than before step n°1
Hope it helps