The best explanation I can find of check spring function is from the Navy manual:  The check spring prevents the needle from stabbing into it's own thread by keeping tension on the thread until the needle tip buries into the material.  Just when the needle eye reaches the goods, the check spring should land on it's stop, thereby relieving tension on the thread just as the takeup lever begins its down stroke.   If my observations are correct, the check spring should not enter the game again