Each bar is expanded into rows of prices, with buy and sell volume printed at each. It turns one number, bar volume, into a distribution, revealing where inside the bar the activity actually happened.
Traders use it to spot absorption, where heavy aggressive buying at a price produces no upward progress, and imbalances, where one side substantially outweighs the other at a price. This is the highest-resolution version of effort-vs-result.
It demands tick data, a lot of screen space, and considerable practice, and it encourages over-reading noise at the smallest scale. It also tells you nothing about what will happen next; it describes what just did, in detail.
Related: cumulative-delta, market-profile, time-and-sales, effort-vs-result, volume-profile