Ask Volumes in the level 2 are much larger then volumes at the bid. Positive. The stack is the number of M&Ms at the same ask & bid price.
EX; there are 3 M&Ms at highest ask with a total of 18 mill and there are 4 at highest bid for 5 mill. 18 mil want to sell and 5 mil want to buy. Closing the 3 M&M's at ask, will produce 18 mil in trade fees. So chase the ask. Which raises the price.
This is how the big guy darkside creates runs also. They can place large sell orders at ask or just spend some cash and buy up the ask stack themselves. Which ever fits their plan best. This manipulates the price higher, and higher. Till the retail herd takes control from M&Ms, front trading the ask stack and the run is on. Big guys manipulate, M&Ms control. The retail herd, trading without a plan, trades the emotion presented to them. Up or down. Led by the nose like cattle or sheep.
In the case of a fast moving manipulated run. M&Ms will chase the volume for increased trading fees. The larger the difference, the more brazen the M&Ms will go naked chasing fees. As the difference tightens, they feel time to reverse will create selling, hopefully in a panic. And a chance to cover their naked short position. All the time closing open market orders first, then their client book, for small arbitrage gains and continued trade fees, seen with continued down side emotion.