In order to create a short position, you need to lock collateral assets to Mirror Protocol, and mint new mAssets. Unlike Borrow positions, newly minted assets are immediately sold at Terraswap price for UST. UST returned for shorting mAssets will be locked for a defined lock_period. To close your short position, you must first hold shorted amount of mAssets. These mAssets will be burned to return your locked collateral.