It's on all the tables in the Turbo-Tuner. So you get the map trace on the boost table, main fuel table, full throttle fuel table, and the ignition table. The video clip shows it working on the main fuel table, where x = rpm, y = engine load. The car is stationary in neutral, so the yellow box is following the engine revs at almost zero load.
What is map trace? Well basically it is an indicator (yellow box) which tells you exactly whereabouts in the table your engine is running at any moment in time. It is called map trace because it effectively traces the position on the map, just like satnavs display your position on a road map.
Why is it important / useful? At closed throttle and at full throttle it is easy to work out where you are in the table, because you are either at one end (0%) or the other end (100%). The problem arises when you want to make adjustments at part throttle. Without map trace it is pure guess work because you don't know if you are at 50% load or 75% load for example. The map trace tells you exactly where you are in terms of load and rpm so you can see immediately which values in the table you should change, if required.
In summary, the map trace feature allows you to fine tune the full range of the engine's performance envelope much more quickly and accurately, rather than just focusing on a rough-guess full throttle tune.
Whether it is useful to you depends on how finely you want to tune. For coarse tuning with very few mods it's probably not going to be of much benefit. But to get the most out of a heavily modified setup, it will be extremely useful.
