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There's maximum brake torque timing for every operation point. Your goal is to be at that point all the time. Stock timing maps are often slightly retarded from MBT timing, so that's why we say more advance is usually better, but up to a point obviously.
On my TDI I advanced timing quite a lot since stock it was retarded (more advance means more pressure and more stress on the components), and I run higher than stock EGR rates, which slows the burn rate, and the ECU was not compensating for it.
On recent gassers with knock sensors timing is often kept at the most advance possible by the ECU, but then again I'd like to have a switched resistor on the IAT sensor for low load cruising to see if I could gain a degree or two. There are no huge gains to be had, but every bit counts.
If you just look at the ING value you can almost think of it as a vacuum gauge, or for a given operating point it can serve as a diagnosis test. Generally speaking, all else being equal, it changes proportionally to RPM, and inversely proportional to load.
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