bookworm - I do indeed mean 'DTK' and NOT 'TRK'! The idea is that you should check the GPS value against the planned value at the turning point to show any gross error. The mini-CDI will show cross-track error which can be very useful if you have to deviate from track for some reason - such as a rain shower, for example.
A very common mistake is to have 'DTK' and 'TRK' displayed. All that will do is to tell you (from a rather inaccurate GPS-calculation of track angle) that, when the values are equal, you are parallel to your desired track, not maintaining it!