How and when are the hands on a watch locked into sinc w/auto movement?

[cjcar631]

The reason I ask is because I, as well as others, were experiencing what appeared to be the same problem with a certain brand watch. Shortly after purchasing it the movement was gaining/losing large incriments of time (minutes/day). At first I thought what most people were thinking... just a cheap crappy movement. But upon closer inspection I realized the minute hand was not locked into position even with the crown fully secured. All it took was a good shake of the wrist and the minute hand would move forward/backwards up to 5, 10, and even as much as 15 minutes at a time. After returning it for a new one I've had no such problems and the watch, although identical in all respects (exhibition case back), keeps great time. What would cause this?