Bidwacking. This is when a seller chooses to sell his/her position 'at market', instead of 'at ask'.
Depending on the size of the position, the shares will 'hit the bid' and usually have the effect of causing the bid to drop quickly. Buyers suddenly realize that sellers are anxious to sell, and as a result, they drop their "bid" price even further. Buying and selling is all a matter of creating a market perception. most people want a stock that is in high demand, where sellers are reluctant to sell except at a higher price. Similarly, Most buyers will get spooked when many sellers suddenly are willing to drop their price just to get out of the stock.
An excellent point. I think some investors have the erroneous idea that if they enter a limit order that they are divulging private information to some market maker who will ensure that the stock never reaches their level. So they resort to hitting bids or lifting offerings and end up costing themselves money.
Rarely is a market order the optimal choice.
