In general, there’s one more stride if you trot, rather than canter, in.
As for the exercise, it’s more basic. When you trot a fence (properly), there’s no real distance, as the trot is a two-beat gait. When you canter in (three beat) there’s a distance, which can be short, long, or just right. If your (canter) distance in to a line is either short or long, you will need to adjust stride in the line (going forward or holding) to get the determined number of strides out.
It’s difficult to know, without more information, what skill(s) the clinician was trying to work on in your case. Sometimes it’s patience - it can much harder to actually trot the “in” of a line.