Say the bid is .0005 and the ask is .0006. When someone on the bid gives in and decides to buy at .0006 that is considered a buy. If on the other hand the bidder was able to hold his ground and forces the guy on the ask to to sell at .0005 its considered a sell. It's used to reveal the psychology and momentum of the stock.
If you are looking at the Ihub trades screen, then it is not exactly correct. Many of my trade buys at the bid show up as trade sells instead. I am sure that I am not the only one who has experienced this.
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