by certing you are physically removing shares from the float. example: If you are with Etrade now, they can use your shares to lend to someone else to short with. Much like a bank, the $100 bill you give them to put in your account will not be the same $100 bill you get back when you withdraw. Stocks are all electronicaly credited to you, but the physical share inventory is still held by the broker. When a run happens the brokers have to have enough inventory to cover, in case all the owners sell. They have three days to get them from the open market after crediting to your account. If there is a shortage of shares, or the price is too high, then the GAMES begin as they try to buy back the shares they need to stay legal with the SEC.
Market Data 
Markets 
AI
