The basic premise is not mathematical at all, but rather almost psychological. When you are watching a company's chart go up and down, sometimes there will be a gap in prices - for example, take a look at a candle chart of WAMKQ and you will see that recently there was a Gap in traded prices.
Lets say people are buying and selling at $0.90. There are trades at $0.91, $0.92, and as the days go by, it continues like so:
100 shares bought at $0.93
100 shares bought at $0.94
100 shares bought at $0.95
100 shares bought at $0.96
100 shares bought at $0.97
100 shares bought at $0.98
100 shares bought at $0.99
Then some news comes out, and people get excited, and will purchase the stock at market value because they don't want to set a limit price on a stock that they think is about to go sky high. So if the BID/ASK for the above said stock just after the last trade (100 @ $0.99) is $1.00 (bid), $1.50 (ask), and someone puts in a trade at market price, the stock price now has gone from $0.99 during the last trade to $1.50, creating a $0.51 gap in PPS ($1.50 - $0.99).
Even if the stock continues to go up ($1.51, $1.52, $1.53, etc), statistically the share price will go back to the gap (between $0.99 and $1.50) to fill it. Watch how stocks jump up and down, and you will see that after a gap in PPS the share price usually dips back down later on down the road to "fill" the gap.
Hope that helps.
