As is typical for a trial production stage, this initial run will be small-scale, and designed to identify any potential problems in the watch so that they can be fixed prior to the main production period. It usually occurs just before mass production begins, so if true, this news could be a very good indication that the release of the second-generation Apple Watch could be happening in the next few months.
This would also tie in with comments made by Quanta’s chairman last November, when he said that the second-generation Apple Watch would be on sale by the end of the second quarter of 2016.
Source: Rumor: Second-gen Apple Watch going into trial production this month