“W” trades are almost always exclusive to large buy orders. The buyer and the MM explicitly agree as to how the order will be executed, especially if the order is for a large number of shares. For example, upon receiving a market order from the buyer, the MM and the customer agree that the MM will execute the order in pieces throughout the day, with the expectation that this “working” of the order will result in a price to the customer that is superior to the price at which the stock is trading at the time of the the order.
It's a way to avoid a runup in price when placing larger buys
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