The Flexible Durso Standpipes are made from flexible Schedule 40 PVC. This PVC has a slight give to it over some distance. Short distances such as 12 inches (30.5 cm) will not have much flex at all. Essentially the flexible PVC comes with a built in curve from how it was rolled and shipped which comes in handy for small overflow chambers.
The curve lets the Tee/Elbow assembly be off-center from the bulkhead. This curve is only helpful in tight spaces, it is of no help in larger overflow chambers as the standpipe will simply lean over to the side.
Flexible PVC is more flexible when heated. This would include soaking in boiling water or heated with a hot air gun. This can be used to enhance the bend or even straighten the pipe should a curve no longer be needed.