Works all the time when a stock closes strong on a Friday. Also, some companies issues some juicy PRs over the weekend. Bottom-line, investors and momo players usually find out about these kind of stocks over the weekend coz they were busy in other stocks. So, most people, especially the newbies, put a bid to purchase during the weekend or Monday morning before the market opens. Of course their orders will be queued until the next trading day (Monday)> 90% of all times, the MMs will choose to fill the orders that will give them the highest profit from the BID & ASK spread upon opening on Monday morning. Then they will come down immediately on the BID, after filling the high-priced orders. They will want fill the ones at the bottom. Experienced traders usually know at what price to buy at (support level) and thus the MMs usually will come down on the BID in order to fill these orders (that's what we call a gapper)i.e, excited folks chasing to buy, in fear that when it goes up, it will not come down and thus miss out. Closing the gapper is when the MMs get back to the normal trading ranges.
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