This discussion comes up every so often, so I thought I’d show the difference visually.
First, winding yarn into a center-pull ball nearly always incurs at least some tension on the yarn and engaging at least some of its natural elasticity. If yarn is kept at tension, not allowed to relax, its natural elasticity is lost and it’s not always restored with a simple washing and blocking.
When you mount your yarn onto a swift, and then release whatever ties are in place, and lay the end of the yarn into the slot on your ball winder, no matter how gently or slowly you crank the handle, you are still pulling yarn (creating tension) AND pulling the swift round and round (more tension).
This first shot is a full hank of Knitterman’s Lace, taken off my wooden umbrella swift. It is neat, but quite firm. Notice the little circular lines on the white base beneath the ball. You see there are two markings visible. And when I took this ball off the winder, the center collapsed into the center because of the tension held within the ball.
So, I put that first ball onto the floor and started winding again, roughly the same speed as before. This time, even though I am pulling the yarn, same as before, the yarn is flowing freely from the ball on the floor, without any additional tension of having to pull the swift round and round. Thus, this ball is not being pulled as tightly as it was the first time. And look at the support base beneath the yarn — you can just barely make out the outer-most little line going around the base.
When I took it off the ball-winder, the center didn’t fill in immediately, but was relaxed enough I had no trouble inserting a little piece of bubble-wrapped foam (see the post from April 1). And even with that inserted into the center, the rest of the ball was relaxed and squishy, not hard.
Ideally you will not convert hanks into center-pull balls until you are actually ready to knit, but if you need to prepare several center-pull balls like this ahead of time for a particular project, make sure you do it twice — once off the swift, and then again from just the balled yarn.