The DTCC, through various brokerages, stops clearing trades for 'free' and charges fees (up to $700 per trade) to the stockholder in the case of a chill. In the case of a lock, the DTCC stops clearing any and all trades.
In the case of JBII, if the DTCC sees unusually high volume, they often chill the stock, or if it's their belief that fraud may be involved.