that would occur when the shares borrowed in order to short the stock are called by the lender of the shares and unless the brokerage firm can find other shares to borrow and replace the loaned shares, it is forced to buy into the market in order to return the borrowed shares to the lender. The brokerage firm sometimes gives the short seller about 1/2 day to buy the shares in the market themselves before the brokerage firm does the forced buyin. The forced buyin sometimes is priced at the vwap for the day, but unfortunately for the short seller it is sometimes done at whatever price the brokerage firm buys the shares in the market, which sometimes means hitting the increasing ask in low float stocks.
Please describe what would precipitate what you refer to as a forced buy-in.
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