(originally posted on Thingiverse, moved here for greater coherency and minor updates.)
A swift holds a hank of yarn while it is being wound into a ball, so you don’t have to fight it all the way.
If you’re a hardcore knitter, you buy your yarn from hippy communes in faraway places, or you spin it yourself. This gives you yarn in hanks – long loops that are good for dyeing, but non-trivial to knit with. To get it into a ball, you need someone or something to hold it and prevent tangles, while you do the winding by hand or with a ball winder. That’s where the swift comes in: it holds the yarn, and rotates gently on demand, allowing to you to unwind/rewind yarn as needed without requiring a large family to assist you.
The canonical example of a swift is the umbrella swift.
We wanted something similarly light, yet sturdy enough for the wildest of knitting parties. After extensive tea, we converged on a design that uses scissoring arms to expand and contract. A bit of CAD and lasers later, we had this:
Three arms are mounted on a spindle that rotates around a threaded rod. It can rotate freely, but has sufficient dampening to not overshoot – it just feeds you yarn on demand. Cable ties make good hinges, apparently.
And it spins: