When most people talk about neck tension, they are usually talking about bushing dies. A bushing die usually unscrews around the top, and a neck sizing bushing slide into the bushing, controlling the amount of neck tension placed on the case.
You are referring to the expander button (Balls.), that is pulled through the case neck when sizing, and majority of bushing dies do not have such a thing.

In a standard die they usually have a large amount of sizing on the case neck (Over size.) and then the expander button opens the case up to a proper size where it will still have tension and firmly hold the bullet. There is a lot of information about expanding balls, take it for what it is worth. Most common belief is that the cause concentricity problems that lead to accuracy problems. The expander ball pulls the neck out of shape, when it pulls back though. Many people remove them, from their dies, some of your better die (Forster.) they unscrew, and can be changed to different sizes.

More than likely what you did was polished out the expander button, cause it to have less friction, and causing less disruption on the case resulting in better accuracy. Depending on how much you took off of it you probley lessoned the amount of tension placed on the round; which can also have several affects. If you are jamming the bullet it would have less tension allowing the bullet to seat back further.

When someone talks about neck tension, they are talking about how much they re underszing the case neck. A simple description would be as this. Using the 6mm PPC as a model most BR shooters have a undersized chamber in there rife, and trim the brass to fit. Most PPC shooters use a .262 neck. So what they do is take the brass where it measures .012 out of the box, and cut it down to where it measures .0095. You take the bullet diameter of .243 and you add both sides of the case measurement (.243+ .0085 + .0085=) and get a total measurement of .260. (This gives then a .002 clearance in the chamber.) There loaded round will measure .260. Now if they was to size that using a .260 bushing they would have 0 neck tension. Now most competitive BR shooters use a .257 sizing bushing, giving them .003 neck tension on the round. (.260-.257= .003) If they go down to a .255 bushing they now have .005 neck tension on the round.