I am guessing that at the beginning of the cast is a bug/glitch rather than near or at the end where latency and lag come into play. I know with my connection i tend to be able to start moving again at about 90% of the cast. This timing gets worse with lag.

Rather than at the beginning which could easily be exploited in raid and such. So the wording might make sense. Can't really fix a lag issue but if its exploit that's completely different.