The Greek letter Φ (Phi) is used to represent the golden ratio also known as the Fibonacci ratio. Many things in nature exhibit the Fibonacci ratio Phi. The same is true for systems based upon human behavior such as the stock market where stock prices largely reflect human opinions, valuations and expectations. A study by mathematical psychologist Vladimir Lefebvre demonstrated that humans exhibit positive and negative evaluations in their the opinions in a ratio that approaches phi. The expansion and retracements of equity prices show an affinity for the ratios of 0.382, .5, 0.618, 1, 1.382, and 1.618. Phi-Trader makes use of that property to help you predict to retracement and expansion targets for a stock. Those price points are the sweet spots for entering & exiting trades. Volume used in conjunction with the Fibonacci ratio levels helps confirm whether the trade is likely a good one.
For the full A-B-C-D cycle, the price spread from A-B will equal that for C-D. The B-C retracement can influence this. If B-C was greater than 50% of A-B, then it is likely that C-D will be less than A-B. If B-C is between 38.2% and 50% of A-B, then it is likely that C-D will equal A-B. If the retracement B-C was less than 38.2% of A-B, then C-D may well exceed A-B. As the equity price goes up, eventually it will begin to retrace.
Volumes are also important in confirming things. Still looking at an A-B-C Up, what you want to see are higher volume levels on the up-trend, and decreasing volumes on the retracements. When moving up and passing through the levels of a previous high, you want to see higher volume levels than there were on the day of the previous high. When retracing you want to see the volume dry up, and in particular, if the price drops below a previous swing low, it needs to do so on less volume than on that previous low's day.
This is a very high level description. More complete references on Fibonacci A-B-C's, confluence, and volumes can be found elsewhere on the Internet.