The bookie's spread moves until they get to a number where the "fors" match the "against". In a perfect world one group pay the other and the bookie keeps the Vig for the losing bets.
Perfect day for MM's volume is high...number ticks up and down..and there are just as many buys as sells. MM could care less what the stock price is. They made there money buying X number of bids and selling the same number of buys. The difference is their VIG. The constant tinkering with the stock price is telling you that they are looking to BUY what you are selling. If they had plenty to sell they could care less if it ran wild. As long as volume doesn't swing way down to a ton of selling on the bid...these boys are thin on inventory.
Just my take. Any one have another reason why an MM wants the price to go down?...because I think we can all agree that's obvious. Why would you want the price to go down on a ticker you had a ton of shares you were looking to sell at a profit? Because you can't just sell all day.