im sure DB will explain it in more detail than myself but whats happening when you see volume surges as price rallies is the large players are dumping some of the supply they had accumulated earlier in the process. they do it as price is rallying because they know this is causing the herd to become greedy and they will be buying in fear of missing out. The herds buying creates the liquidity the institutions needs to dump off the large amounts of supply they had accumulated without immediately stopping the up trend..although when the institutions are selling and you see this volume surge in an up trend you can be sure the trend will be ending shortly. As for the volume surge during the down trend what is happening is exactly the opposite. the market is completely bearish and the public is panic selling thinking the stock is going to zero. The institutions, however, realize the end of this down trend is near and absorb all the supply the public just threw on to the market at discounted prices. The down trend may not immediately end after the surge but soon price will form a range and the process of accumulation begins and starts the process all over again.