Order flow isn't a standalone edge but a layer of market data showing patterns in how institutional traders move prices. Recognize recurring patterns in institutional activity and you understand what actually drives price movement.
Order flow is not an edge in and of itself. Order flow is a tool that you need to read and look for patterns in the data you read. Just like with price, it's just a deeper layer of price, let's say, information and data. So you still look for patterns just like a price action trader, but you look for patterns in the institutional order flow, in the order flow made by traders that matter, is the causing factor of price movements.
