I will give you an example how the PIQ can help you.
Imagine you trade a thick market like the ZN. On each tick the bid/ ask size might be around 1000 to 3000 contracts. Lets say you place a limit buy order now, several ticks away from the inside bid/ask. The bid size at the the price where you place your order is 3000. This means 3000 contracts are waiting to be filled here. Now after you place your limit buy order at the same tick, the bid size changes to 3001 contracts and the PIQ value will be 3000, meaning that there are 3000 contracts in front of you in the queue.
Now since you have placed your order a few ticks away from the inside market, you can wait and see what happens. ZN usually does not move alot, so it is possible that hours pass and the market will still not reach your limit price. But pay attention how the PIQ is changing. Even if there are no trades yet at that price level, your position in the queue might have improved significantly, because people and especially the computers are constantly cancelling and readjusting their orders. Sometimes you might get so lucky with your order, that by the time that the market really arrives at your price level, your position in queue has improved from for example from 3000 to 50, with the bid size maybe still around 2000 or 3000 or even more contracts. This means now: once price starts trading at your level, you will almost immediately get filled. The moment you get filled on the bid, place a limit sell order 1 tick higher. Now you have to watch closely. Is there more buying or more selling ? What is the market doing now ? But at this position, you have an edge: you have been filled at the bid, and there are still thousands of contracts behind you. If you see and/or feel that there is too much selling pressure, and there might be a risk that the market will push through the bid, then you can payout your edge and sell the contract you got filled on at the same price, because there is still a longe queue behind you. You scratched the trade.
If you are more lucky, and right after you got filled the buyers start buy into the offer, then you will probably make 1 tick profit with this trade.
You can build a whole trading system around this idea. Get a little edge by a great position in queue, make a few scratches a day instead of a few 1 or 2 tick losing trades, make a few 1 tick winners. In ZN you can trade that way with serious size, so a handful of ticks a day is more than enough to make a great living.
But of course, to be really successful with a trading style like this, you have to spend countless hours in that market and get a feel for how it moves, to know all its characteristics. But there is no need for secret formulas, indicators, fancy charts and all that gimmick stuff. Just a DOM to follow the pure market activity and plenty of time and love for what you do.