This routine should be used manually on single orders when they are clearly not correctly fitted. This will usually be seen by viewing the paths of the fitted order traces using ech_trplt. In many cases it will be possible to get a good trace fit by clipping a set of obviously bad samples, and re-fitting (perhaps with a lower degree of polynomial).
The other main option for coping with such problems is to rely on the trace consistency checking routine. This will produce a good set of consistent trace fits in most cases.
If a particular order is beyond recovery and you do not wish to extract it at all, then it may be disabled by clipping away all of its trace samples with this routine.
Martin Clayton