During regular market hours millions of shares are bought and sold raising and lowering share prices by pennies, but I'm curious why in after hours trading I'll see like 500 shares raise or lower the price by like 10 or 20 cents. Thank you in advance.
It all has to do with liquidity. Supply vs. demand (so to speak), size of bids and offers available on the book vs. size of market orders eating away at them. During after hours, liquidity is thinner and it takes less effort to move price. If you'd like a more in depth answer, I've already explained the process here. Although more FX orientated, it contains the concept you need to answer your question: http://www.forexfactory.com/showthread.php?p=7957450#post7957450 Specifically in the DOM price discovery process of part 2 you will find the answer.