its when you make a grid with price on the y-axis and the volume on the x-axis. For a series of days you plot each day as a point by volume and price and watch how those lines begin to evolve...its a techniqe that allows you to map and discern changes in pps and volume as they together give a more complete view of what's happening with the stock price. when the pattern devoleps into a clockwise loop its considered bullish(counterclockwise = bearish)
as the price moves down and to the right it indicates a decline in pps and an inrease in volume(distribution), then in moves to the left, as volume decreeases(consolidation) before moving up and to the right(beginning signs of accumulation) indicating a strong rise in pps and volume - very bullish.
A great and seldomly used tool for spotting reversals...