To curve to the left, you have ease in extra fabric under the presser foot as you sew. To curve to the right, it helps to pull the fabric, holding it very taut. Obviously this takes a lot of practice, and I've been playing with this now for about 10 years. My favourite fabrics are silk neckties--those little bits of gorgeous fabrics that are already cut on the bias.
So, it's a little different from cutting cotton fabrics in straight lines with a ruler and rotary cutter. Lining up points and seams is not an issue with bias strip piecing--it's quite freeing!
This is the first time I've created a sprial with this technique (I've crocheted spirals, plus I've appliqued a tri-spiral onto the back of stoles) and it's taken a little play time to feel comfortable maniplating several small pieces of fabric at once. However, I'm pleased with the results so far. Here's the spiral I made today for the other end of this stole.
Donna has made an interesting observation about the spiral as a religious symbol. I've never thought of it as particularly religious although a version of it is called a Celtic spiral (from New Grange, Ireland) which I have used on a couple of stoles. I know that it's been carved in rocks and walls for eons, and is a common symbol on pottery and textiles around the world. The spiral may well mean different things in different cultures. It is probably a wonderfully universal symbol!